Train reversing into mobile unit, erroneously (7)
I believe the answer is:
retinue
'train' is the definition.
(people following someone in a procession)
'reversing into mobile unit erroneously' is the wordplay.
'reversing' is a reversal indicator.
'into' indicates the answer is hidden within the clue.
'euniter' is hidden in the letters of 'mobile unit erroneously'.
'euniter' reversed gives 'RETINUE'.
(Other definitions for retinue that I've seen before include "Group around VIP" , "Train; attendants" , "Group attending a person" , "Body of attendants" , "Suite of followers" .)