Fastener lacking in a zip for Americans? (3)
I believe the answer is:
nil
'zip for americans?' is the definition.
(zip can mean zero or nil in US English)
'fastener lacking in a' is the wordplay.
'fastener' becomes 'nail' (nail is a kind of fastener).
'lacking in' suggests deleting specific letters.
'nail' with 'a' taken out is 'NIL'.
(Other definitions for nil that I've seen before include "Zero, no score" , "none" , "0" , "You get nothing" , "Nothing, zilch" .)