Mars mission within months (5)
I believe the answer is:
maims
'mars' is the definition.
(mar can mean to harm or maim)
'mission within months' is the wordplay.
'mission' becomes 'aim' (I've seen this before).
'within' indicates putting letters inside.
'months' becomes 'ms' (abbreviation for months).
'aim' going within 'ms' is 'MAIMS'.
(Other definitions for maims that I've seen before include "Cripples" , "causes permanent hurt" , "Makes defective" .)