In discussion solitary makes advance (4)
I believe the answer is:
loan
'advance' is the definition.
(I've seen this before)
'in discussion solitary' is the wordplay.
'in discussion' indicates a 'sounds like' (homophone) clue.
'solitary' becomes 'lone' (synonyms).
'lone' sounds like 'LOAN'.
'makes' acts as a link.
(Other definitions for loan that I've seen before include "Advance, borrowing" , "It's given temporarily" , "It's given but has to be returned" , "Act of lending" , "Money borrowed at interest" .)