Object after old knight pulled up zip (7)
I believe the answer is:
nothing
'zip' is the definition.
(I know that zip can be written as nothing)
'object after old knight pulled up' is the wordplay.
'object' becomes 'thing' (an object is a thing or item).
'after' says to put letters next to each other.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'knight' becomes 'n' (chess abbreviation).
'pulled up' shows that the letters should be reversed in order.
'o'+'n'='on'
'on' written backwards gives 'no'.
'thing' after 'no' is 'NOTHING'.
(Other definitions for nothing that I've seen before include "signifying love" , "Nought; of no value" , "Not a whit" , "can mean love" , "without object" .)