Guided a returning assistant in state capital (8)
I believe the answer is:
adelaide
'state capital' is the definition.
(state capital of South Australia)
'guided a returning assistant' is the wordplay.
'guided' becomes 'led' ('lead' can be a synonym of 'guide').
'returning' says the letters should be written backwards.
'assistant' becomes 'aide' (aide is a kind of assistant).
'led'+'a'='leda'
'leda' backwards is 'adel'.
'adel'+'aide'='ADELAIDE'
'in' is the link.
(Other definitions for adelaide that I've seen before include "Australian capital" , "State capital of South Australia" , "Queen to William IV - setting for many an 6 test" , "town" , "Lead aide out to Australia" .)