Surrounded by four characters in crowd, I marched back (4)
I believe the answer is:
amid
'surrounded by' is the definition.
(amid means among or surrounded by)
'four characters in crowd i marched back' is the wordplay.
'four characters in' indicates a hidden word (four letters hidden in the clue).
'back' says the letters should be written in reverse.
'dima' is hidden in the letters of 'crowd i marched'.
'dima' reversed gives 'AMID'.
(Other definitions for amid that I've seen before include "In the centre of" , "Maid (anag)" , "In the course of" , "A dim sort of preposition" , "Surrounded by" .)