Injure top of head and limb (4)
I believe the answer is:
harm
'injure' is the definition.
(harming is a kind of injuring)
'top of head and limb' is the wordplay.
'top of' indicates taking the first letters.
'and' means one lot of letters go next to another.
'limb' becomes 'arm' (arm is a kind of limb).
The initial letter of 'head' is 'h'.
'h'+'arm'='HARM'
(Other definitions for harm that I've seen before include "Injure, injury" , "Damage, hurt" , "Injure or damage" , "Hurt, injure" , "See 13" .)