Instrument socially acceptable, possibly left out (5)
I believe the answer is:
flute
'instrument' is the definition.
(flute is a kind of instrument)
'socially acceptable possibly left out' is the wordplay.
'socially acceptable' becomes 'u'.
'possibly' indicates anagramming the letters.
'out' indicates putting letters inside.
'left' is an anagram of 'flte'.
'u' put into 'flte' is 'FLUTE'.
(Other definitions for flute that I've seen before include "High-pitched woodwind instrument" , "Blown" , "See 20" , "Wind instrument" , "Musical instrument - wineglass" .)