Assistant follows lead criminal in state capital (8)
I believe the answer is:
adelaide
'state capital' is the definition.
(state capital of South Australia)
'assistant follows lead criminal' is the wordplay.
'assistant' becomes 'aide' (aide is a kind of assistant).
'follows' says to put letters next to each other.
'criminal' indicates an anagram.
'lead' anagrammed gives 'adel'.
'aide' put after 'adel' is 'ADELAIDE'.
'in' is the link.
(Other definitions for adelaide that I've seen before include "somewhere in Australia" , "Australian city named for William IV's queen" , "old consort" , "19th century queen" , "State capital of South Australia" .)