Should have purchased missing top (5)
I believe the answer is:
ought
'should' is the definition.
(synonyms)
'purchased missing top' is the wordplay.
'purchased' becomes 'bought' ('buy' can be a synonym of 'purchase').
'missing top' means to remove the first letter.
'bought' with its initial letter taken off is 'OUGHT'.
'have' acts as a link.
(Other definitions for ought that I've seen before include "outdated anything at all?" , "Has a duty (to do something)" , "Be logically necessary" , "Strange thing" , "Verb implying duty; zero, nothing (archaic)" .)