Solitary-sounding advance (4)
I believe the answer is:
loan
'advance' is the definition.
(an advance is a loan)
'solitary-sounding' is the wordplay.
'solitary' becomes 'lone' (synonyms).
'sounding' shows a homophone (sound like).
'lone' is a homophone of 'LOAN'.
(Other definitions for loan that I've seen before include "Overdraft" , "Advance (money)" , "Cash advance" , "Mortgage" , "temporary payment" .)