Supply deal secured by helper making capital (8)
I believe the answer is:
adelaide
'capital' is the definition.
(state capital of South Australia)
'supply deal secured by helper' is the wordplay.
'supply' indicates an anagram (in a supple way).
'secured by' is an insertion indicator (inserted letters are held or secured).
'helper' becomes 'aide' (aide is a kind of helper).
'deal' anagrammed gives 'dela'.
'dela' put into 'aide' is 'ADELAIDE'.
'making' is the link.
(Other definitions for adelaide that I've seen before include "19th century queen" , "Antipodean city" , "Commonwealth city" , "Queen < in opera" , "Lead aide out to Australia" .)