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' (similar in meaning).
'setting for' indicates putting letters inside (inserted letters are set inside others).
'a hundred' becomes 'c' (Roman numeral).
'sore' enclosing 'c' is 'SCORE'.
'and' is the link.
(Other definitions for score that I've seen before include "Group of 20" , "Mark; orchestrate" , "Keep count" , "Hit - music" , "Twenty; scratch" .)