Informal evening turned into regret for entourage (7)
I believe the answer is:
retinue
'for entourage' is the definition.
(synonyms)
'informal evening turned into regret' is the wordplay.
'informal evening' becomes 'nite' (I've seen this in another clue).
'turned' says the letters should be written backwards.
'into' is an insertion indicator.
'regret' becomes 'rue' (to rue something is to regret it).
'nite' backwards is 'etin'.
'etin' put within 'rue' is 'RETINUE'.
(Other definitions for retinue that I've seen before include "Group of attendants for VIP" , "Train; attendants" , "Reunite (anag) - group of servants" , "Cortege" , "Group attending a person" .)