Fish for the trout swimming around Birkenhead (6)
I believe the answer is:
turbot
'fish' is the definition.
(turbot is a kind of fish)
'the trout swimming around birkenhead' is the wordplay.
'swimming' indicates an anagram.
'around' is an insertion indicator.
'birkenhead' becomes 'b' (head letter of 'birken').
'trout' with letters rearranged gives 'turot'.
'turot' placed around 'b' is 'TURBOT'.
'for' is the link.
(Other definitions for turbot that I've seen before include "Large succulent flatfish" , "[SOMETHING FISHY]" , "Large brownish European flatfish" , "Swimmer" , "Pale brown, speckled flatfish" .)