Old money essentially funding upper-class jazzmen (6)
I believe the answer is:
ducats
'old money' is the definition.
(historical type of gold coin)
'essentially funding upper-class jazzmen' is the wordplay.
'essentially' means to look at the middle letters.
'upper-class' becomes 'u' (similar in meaning).
'jazzmen' becomes 'cats' (I've seen this in another clue).
The central letter of 'funding' is 'd'.
'd'+'u'+'cats'='DUCATS'
(Other definitions for ducats that I've seen before include "money" , "Shylock lost" , "Old gold coins" , "pieces of old" .)