Chummy has to pay about fifty smackers (5)
I believe the answer is:
pally
'chummy' is the definition.
(synonyms)
'to pay about fifty smackers' is the wordplay.
'about' is an insertion indicator.
'fifty' becomes 'l' (roman numeral).
'smackers' becomes 'l' (I can't explain this - if you can you should give a lot more credence to this answer).
'l'+'l'='ll'
'pay' enclosing 'll' is 'PALLY'.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for pally that I've seen before include "Very familiar" , "Thick" , "On friendly terms" , "Close, friendly" , "Chummy" .)