Catch on -- shoot! (4)
I believe the answer is:
twig
I believe this clue is a double definition.
'catch on' is the first definition.
('twig' can be a synonym of 'catch on')
'shoot' is the second definition.
(thesaurus)
(Other definitions for twig that I've seen before include "Small piece of wood" , "Suddenly get" , "very little from box, perhaps" , "A small branch - cop on?" , "Small piece of tree" .)