Catapult fish in seconds, very sexy! (9)
I believe the answer is:
slingshot
'catapult' is the definition.
(synonyms)
'fish in seconds very sexy' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish).
'in' indicates putting letters inside.
'seconds' becomes 'ss' (an abbreviation for seconds written twice).
'very sexy' becomes 'hot' (I've seen this in another clue).
'ling' going within 'ss' is 'slings'.
'slings'+'hot'='SLINGSHOT'
(Other definitions for slingshot that I've seen before include "Weapon American kid may produce" , "fatal to Goliath" , "Sing Holst? (Anag)" , "No slights (anag) - catapult (US)" , "enough to knock you out" .)