Soldiers on street -- sister and agent heading back (8)
I believe the answer is:
persists
'soldiers on' is the definition.
(I've seen this before)
'street sister and agent heading back' is the wordplay.
'street' becomes 'st'.
'sister' becomes 'sis' ('sis' can be a synonym of 'sister').
'and' means one lot of letters go next to another.
'agent' becomes 'rep' (short for representative).
'heading back' says the letters should be written in reverse.
'st'+'sis'+'rep'='stsisrep'
'stsisrep' written backwards gives 'PERSISTS'.
(Other definitions for persists that I've seen before include "Continues to exist" , "Soldiers on" , "Carries on, perseveres" , "Continues in attitude or objection" , "Maintains a course against opposition" .)