Requirement to return key was irritating (7)
I believe the answer is:
needled
'was irritating' is the definition.
'requirement to return key' is the wordplay.
'requirement' becomes 'need' ('need' can be a synonym of 'requirement').
'to return' is a reversal indicator.
'key' becomes 'del' (del key on a keyboard).
'del' written backwards gives 'led'.
'need'+'led'='NEEDLED'
(Other definitions for needled that I've seen before include "Provoked or teased with knitting implements? (7)" , "Goaded or provoked" , "Irked" , "was annoying" , "Irritated (sl.)" .)