United regularly hiding section that's unprepossessing (9)
I believe the answer is:
unsightly
'unprepossessing' is the definition.
(thesaurus)
'united regularly hiding section' is the wordplay.
'united' becomes 'U' (abbreviation).
'regularly' becomes 'nightly' (happening every night).
'hiding' indicates putting letters inside.
'section' becomes 's' (legal abbreviation).
'nightly' enclosing 's' is 'nsightly'.
'u'+'nsightly'='UNSIGHTLY'
'that's' acts as a link.
(Other definitions for unsightly that I've seen before include "Displeasing to the eye" , "Ugly" , "wild?" , "Unpleasant to look at" , "Not pleasing on the eye" .)