Mostly steamy number by pop singer is capital (8)
I believe the answer is:
honolulu
'is capital' is the definition.
(state capital of Hawaii)
'mostly steamy number by pop singer' is the wordplay.
'mostly' means to remove the last letter.
'steamy' becomes 'hot' (both can mean erotic).
'number' becomes 'no' (abbreviation).
'by' means one lot of letters go next to another.
'pop singer' becomes 'Lulu' (Scottish singer).
'hot' with its last letter removed is 'ho'.
'ho'+'no'+'lulu'='HONOLULU'
(Other definitions for honolulu that I've seen before include "Hawaiian city" , "US Pacific state capital" , "American port" , "Hawaiian capital" , "location in Hawaii" .)