Strike a root vegetable soundly (4)
I believe the answer is:
beat
'strike' is the definition.
(I've seen this before)
'a root vegetable soundly' is the wordplay.
'a root vegetable' becomes 'beet' (beet is a kind of root vegetable).
'soundly' shows a homophone (sound like).
'beet' is a homophone of 'BEAT'.
(Other definitions for beat that I've seen before include "Thrash, punish physically" , "Tan" , "Pound - defeat" , "Defeat; rhythm" , "Thrash with rhythm?" .)