Fish shoulders, oddly! (5)
I believe the answer is:
soles
'fish' is the definition.
(I know that soles is a type of fish)
'shoulders oddly' is the wordplay.
'oddly' means one should take alternating letters (letters in odd-numbered positions).
The alternate letters of 'shoulders' are 'SOLES'.
(Other definitions for soles that I've seen before include "Fish; bottoms of feet" , "Undersurfaces of feet" , "Shoe bottoms" , "Parts of shoes" , "Undersides of feet" .)