Gap admitting one to underworld knees-up? (9)
I believe the answer is:
disparity
'gap' is the definition.
(thesaurus)
'admitting one to underworld knees-up?' is the wordplay.
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'one' becomes 'I' (Roman numeral).
'to underworld' becomes 'dis' (underworld in classical mythology).
'knees up?' becomes 'party' (knees-up means a party).
'dis'+'party'='disparty'
'i' going inside 'disparty' is 'DISPARITY'.
(Another definition for disparity that I've seen is " Inequality".)