Muscle into a capital place for water sports (9)
I believe the answer is:
aquadrome
'place for water sports' is the definition.
(venue for water sports)
'muscle into a capital' is the wordplay.
'muscle' becomes 'quad' (quad is a kind of muscle).
'into' means one lot of letters goes inside another.
'capital' becomes 'Rome' (capital of Italy).
'a'+'rome'='arome'
'quad' going inside 'arome' is 'AQUADROME'.
(Other definitions for aquadrome that I've seen before include "Water-skiing centre" , "water sports venue" .)