An acceptable standard - and not married! (4)
I believe the answer is:
norm
'an acceptable standard' is the definition.
(I've seen this before)
'and not married' is the wordplay.
'and not' becomes 'nor' (I've seen this before).
'married' becomes 'm'.
'nor'+'m'='NORM'
(Other definitions for norm that I've seen before include "Mean" , "Usual or standard thing" , "Rule; average" , "What is conventional, regular" , "Standard regarded as typical" .)