A single girl out of order (5)
I believe the answer is:
amiss
'out of order' is the definition.
('amiss' can be a synonym of 'out of order')
'a single girl' is the wordplay.
'a single' becomes 'a'.
'girl' becomes 'miss' (I've seen this before**).
'a'+'miss'='AMISS'
(Other definitions for amiss that I've seen before include "Not functioning properly" , "Taking wrong turn" , "Unsuitable" , "Wrorg" , "Mistakenly" .)