Zip (n): object next to middle of groin (7)
I believe the answer is:
nothing
'zip' is the definition.
('nothing' can be a synonym of 'zip')
'n object next to middle of groin' is the wordplay.
'object' becomes 'thing' (thing is a kind of object**).
'next to' means one lot of letters go next to another.
'middle of' means to look at the middle letters.
The central letter of 'groin' is 'o'.
'thing' put after 'o' is 'othing'.
'n'+'othing'='NOTHING'
(Other definitions for nothing that I've seen before include "Zilch, nada" , "Zero, naught" , "what House of Peers did particularly well?" , "trivial event" , "One of those sweet sayings" .)