der Happen
bite / morsel
A small piece of food or a bite, often as part of a meal or as a small snack.
Ich habe seit dem Frühstück nur einen kleinen Happen gegessen.
Since breakfast, I've only eaten a small bite.

bite / morsel
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bite / morsel
A small piece of food or a bite, often as part of a meal or as a small snack.
Ich habe seit dem Frühstück nur einen kleinen Happen gegessen.
Since breakfast, I've only eaten a small bite.

You can recognize the literal meaning from the context of food and drink. The figurative meaning usually refers to economic contexts such as contracts, markets, or profits and describes a share.
Er aß nur einen kleinen Happen von dem Kuchen.
He ate only a small bite of the cake.
In the meaning 'a piece of food', 'Happen' (bite/morsel) is rather colloquial. It is often used for a small snack or a quick, simple meal in between.
Vor der langen Fahrt sollten wir noch einen Happen essen.
Before the long drive, we should have a bite to eat.
In the figurative sense, a 'Happen' (piece/share) is a share of something valuable, for example a deal or profit. The phrase 'einen Happen vom Kuchen bekommen' (to get a piece of the pie) is often used.
Jeder Investor wollte einen großen Happen von dem profitablen Geschäft.
Every investor wanted a large piece of the profitable deal.
piece
'das Stück' and 'der Happen' can both mean a small part of food. 'das Stück' is neutral and can be large or small, for example 'ein Stück Brot' (a piece of bread), 'Käse' (cheese), or 'Kuchen' (cake). 'der Happen' more often means a small amount of food that you eat quickly or can eat in one bite. The words can be confused because a 'Happen' is also a piece of food. If you are simply naming a part of something, 'Stück' is safer. If you want to emphasize the small amount of food or the quick bite, 'Happen' sounds more natural.

Vor dem Termin isst Tom nur einen kleinen Happen.
Before the appointment, Tom eats only a small bite.
Lisa legt ein Stück Käse auf das Brot.
Lisa puts a piece of cheese on the bread.