PIN's missing a zero (3)
I believe the answer is:
nil
'zero' is the definition.
(synonyms)
'pin's missing a' is the wordplay.
'pin' becomes 'nail' (I've seen this before).
'missing' suggests deleting specific letters.
'nail' with 'a' taken away is 'NIL'.
(Other definitions for nil that I've seen before include "Nothing - in Latin" , "Nothing, zero" , "Love" , "Score of nought" , "Nothing, no score" .)