Left uncle out - what a blow! (4)
I believe the answer is:
slam
'what a blow' is the definition.
(both can mean a hit)
'left uncle out' is the wordplay.
'left' becomes 'l' (common abbreviation).
'uncle' becomes 'sam' (Uncle Sam is an example).
'out' is an insertion indicator.
'l' placed within 'sam' is 'SLAM'.
(Other definitions for slam that I've seen before include "Shut hard" , "Ambitious contract" , "Close noisily" , "Violently strike against" , "Taking of twelve or thirteen tricks (bridge)" .)