Hit by small three-wheeler (6)
I believe the answer is:
strike
'hit' is the definition.
(I know that hit can be written as strike)
'by small three-wheeler' is the wordplay.
'by' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'three-wheeler' becomes 'trike' (tricycle).
's' put next to 'trike' is 'STRIKE'.
(Other definitions for strike that I've seen before include "Agree on" , "Walk-out" , "success for bowler" , "Hit; cancel" , "Hit; withhold labour" .)