Regret about arranging it en suite (7)
I believe the answer is:
retinue
'suite' is the definition.
('retinue' can be a synonym of 'suite')
'regret about arranging it en' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'about' means one lot of letters goes inside another.
'arranging' indicates anagramming the letters.
'it'+'en'='iten'
'iten' with letters rearranged gives 'etin'.
'rue' enclosing 'etin' is 'RETINUE'.
(Other definitions for retinue that I've seen before include "Train; attendants" , "Entourage, followers" , "Personal staff" , "Reunite (anag) - group of servants" , "Train, escort" .)