Fish for the trout swimming around Birkenhead (6)
I believe the answer is:
turbot
'fish' is the definition.
(type of fish)
'trout swimming around birkenhead' is the wordplay.
'swimming' indicates an anagram (letters swim around into a new order).
'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 the' 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" .)