Immersed in stuff I'd raced around capital (7)
I believe the answer is:
cardiff
'capital' is the definition.
(I know that Cardiff is a capital)
'immersed in stuff i'd raced around' is the wordplay.
'immersed in' indicates a hidden word.
'around' shows that the letters should be reversed in order.
'ffidrac' can be found hidden inside 'stuff id raced'.
'ffidrac' backwards is 'CARDIFF'.
(Other definitions for cardiff that I've seen before include "Welsh city" , "Capital of Wales" , "Capital on the Taff" , "City on the Taff" , "Welsh capital" .)