Injure limb after start of holiday (4)
I believe the answer is:
harm
'injure' is the definition.
(harming is a kind of injuring)
'limb after start of holiday' is the wordplay.
'limb' becomes 'arm' (arm is a kind of limb).
'after' means one lot of letters go next to another.
'start of' indicates taking the first letters.
The initial letter of 'holiday' is 'h'.
'arm' put after 'h' is 'HARM'.
(Other definitions for harm that I've seen before include "Injure, injury" , "Detriment" , "Injure or damage" , "See 13" , "Wound" .)