Bullet needs little time -- fast mover (6)
I believe the answer is:
tracer
'bullet' is the definition.
(thesaurus)
'little time fast mover' is the wordplay.
'little time' becomes 't' (abbreviation for time).
'fast mover' becomes 'racer' (I've seen this before).
't'+'racer'='TRACER'
'needs' acts as a link.
(Other definitions for tracer that I've seen before include "Bullet whose track can be seen - element tracking radiation" , "He's managed to find" , "One finding" , "This bullet leaves smoke in its passage" , "beast to tow" .)