Sid backs company for record entertainment (5)
I believe the answer is:
disco
'record entertainment' is the definition.
(entertainment involving playing records)
'sid backs company' is the wordplay.
'backs' is a reversal indicator.
'company' becomes 'co' (abbreviation).
'sid' reversed gives 'dis'.
'dis'+'co'='DISCO'
'for' is the link.
(Other definitions for disco that I've seen before include "equipment for playing recordings" , "Style of music/dancing" , "Dancing to records" , "One might get down to this" , "Dance club with recorded music" .)