Head of union eaten by red fish (5)
I believe the answer is:
trout
'fish' is the definition.
(I know that trout is a fish)
'head of union eaten by red' is the wordplay.
'head of' says to take the initial letters.
'eaten by' indicates putting letters inside.
'red' becomes 'trot' (I've seen this before).
The initial letter of 'union' is 'u'.
'u' going within 'trot' is 'TROUT'.
(Other definitions for trout that I've seen before include "air" , "Oily fish" , "interfering old woman" , "Performance by five players" , "Tickled fish?" .)