Gents at sea capturing one small seal (6)
I believe the answer is:
signet
'small seal' is the definition.
(type of official seal)
'gents at sea capturing one' is the wordplay.
'at sea' indicates anagramming the letters.
'capturing' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'gents' anagrammed gives 'sgnet'.
'sgnet' placed around 'i' is 'SIGNET'.
(Other definitions for signet that I've seen before include "Tinges (anag) - seal" , "Finger-ring with engraved design" , "Small seal - type of ring" , "Ingest in ring" , "'Seal, on a ring say (6)'" .)