Painful setting for a hundred and twenty (5)
I believe the answer is:
score
'twenty' is the definition.
(synonyms)
'painful setting for a hundred' is the wordplay.
'painful' becomes 'sore' (synonyms).
'setting for' means one lot of letters goes inside another.
'a hundred' becomes 'c' (Roman numeral).
'sore' going around 'c' is 'SCORE'.
'and' acts as a link.
(Other definitions for score that I've seen before include "Group of 20" , "Mark; orchestrate" , "Keep count" , "Hit - music" , "Twenty; scratch" .)