Rich old girl to make bet? (7)
I believe the answer is:
dowager
'rich old girl' is the definition.
(I've seen this before)
'make bet?' is the wordplay.
'make' becomes 'do' (synonyms).
'bet?' becomes 'wager' ('wager' can be a synonym of 'bet').
'do'+'wager'='DOWAGER'
'to' acts as a link.
(Other definitions for dowager that I've seen before include "Widow, imposing elderly woman" , "Wordage for the widow of a peer" , "Woman who holds title from dead husband" , "Dignified (or wealthy) elderly woman" , "Rich old woman" .)