Southern sailors going north catch first of piranha fish (6)
I believe the answer is:
sprats
'fish' is the definition.
(sprats are a type of fish)
'southern sailors going north catch first of piranha' is the wordplay.
'southern' becomes 's' (abbreviation).
'sailors' becomes 'tars' (tar is a kind of sailor).
'going north' says the letters should be written in reverse.
'catch' means one lot of letters goes inside another.
'first of piranha' becomes 'p' (1st letter of 'piranha').
's'+'tars'='stars'
'stars' written backwards gives 'srats'.
'srats' placed around 'p' is 'SPRATS'.
(Other definitions for sprats that I've seen before include "Small marine fish of the herring family" , "Small fish used to catch larger ones" , "'Fish, like small herrings (6)'" , "Small fish, set to catch whales" , "Couple fussy about meat" .)