With a fish right in the beak (5)
I believe the answer is:
brill
'with a fish' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'right in the beak' is the wordplay.
'right' becomes 'r' (abbreviation).
'in the' is an insertion indicator.
'beak' becomes 'bill' (synonyms).
'r' placed into 'bill' is 'BRILL'.
Can you help me to learn more?
(Other definitions for brill that I've seen before include "swimmer" , "catch, perhaps" , "First-class (informal) - fish" , "marvellous" , "Champion" .)