Solitary advance, we hear (4)
I believe the answer is:
lone
'solitary' is the definition.
(synonyms)
'advance we hear' is the wordplay.
'advance' becomes 'loan' (an advance is a loan).
'we hear' indicates a 'sounds like' (homophone) clue.
'loan' sounds like 'LONE'.
(Other definitions for lone that I've seen before include "The ... Ranger, fictional masked cowboy" , "Companionless" , "Single and isolated" , "Unaccompanied" , "Solitary, sounds like advance" .)