Collar's missing a zip (3)
I believe the answer is:
nil
'zip' is the definition.
(both can mean zero)
'collar's missing a' is the wordplay.
'collar' becomes 'nail' (synonyms).
'missing' is a deletion indicator.
'nail' with 'a' removed is 'NIL'.
(Other definitions for nil that I've seen before include "Lacking value" , "Zero, no score" , "Nothing, no score" , "Nothing, zilch" , "Nothing, no value" .)