Come together in backing retinue (5)
I believe the answer is:
unite
'come together' is the definition.
(to unite is to come or join together)
'in backing retinue' is the wordplay.
'backing' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'etinu' can be found hidden inside 'retinue'.
'etinu' backwards is 'UNITE'.
(Other definitions for unite that I've seen before include "fuse" , "Club together" , "Join forces" , "Marry" , "Come together as allies or comrades" .)