Sailor's capsizing holding head of pilot fish (5)
I believe the answer is:
sprat
'fish' is the definition.
(sprat is a kind of fish)
'sailor's capsizing holding head of pilot' is the wordplay.
'sailor's' becomes 'tars' (tar is a kind of sailor**).
'capsizing' is a reversal indicator.
'holding' is an insertion indicator.
'head of' indicates taking the first letters.
The initial letter of 'pilot' is 'p'.
'tars' back-to-front is 'srat'.
'srat' enclosing 'p' is 'SPRAT'.
(Other definitions for sprat that I've seen before include "Young herring" , "Small food fish" , "Small herring-type fish" , "'Set a . . . . . to catch a herring'" , "Small sea fish" .)