Top hairdo in socially acceptable situation (9)
I believe the answer is:
uppermost
'top' is the definition.
('uppermost' can be similar in meaning to 'top')
'hairdo in socially acceptable situation' is the wordplay.
'hairdo' becomes 'perm' (perm is a kind of hairdo).
'in' indicates putting letters inside.
'socially acceptable' becomes 'u'.
'situation' becomes 'post' ('post' can be a synonym of 'situation'**).
'u'+'post'='upost'
'perm' put inside 'upost' is 'UPPERMOST'.
(Other definitions for uppermost that I've seen before include "Highest in place or rank" , "get to top" , "At or nearest to the top" , "On top or first in the mind" , "Highest or most prominent" .)