Mend set, in pieces, broken by American –- I can't afford not to (5,4)
I believe the answer is:
needs must
'i can't afford not to' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'mend set in pieces broken by american' is the wordplay.
'in pieces' is an anagram indicator.
'broken by' indicates putting letters inside (inserted letters break into the word).
'american' becomes 'US' (abbreviation for United States).
'mend'+'set'='mendset'
'mendset' anagrammed gives 'needsmt'.
'needsmt' going around 'us' is 'NEEDS MUST'.
Can you help me to learn more?
(Other definitions for needs must that I've seen before include "what wine-making does" , "Of necessity" , "It can't be helped" , "there's no alternative" , "'. . . . . . . . . when the devil drives'" .)