They wear three-cornered hats and can hold a fair amount of beer? (4-4)

'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps they are linked in a way I don't understand?

'wear three-cornered hats and can hold a fair amount of beer?' is the wordplay.
I cannot quite understand how this works, but
'amount' could be 'jug' (jug is a kind of amount) and 'jug' is found in the answer.
'of' could be 'o' and 'o' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?