Son is bust, forced to eke out an existence (7)
I believe the answer is:
subsist
'eke out an existence' is the definition.
(I've seen this before)
'son is bust forced' is the wordplay.
'son' becomes 's'.
'forced' indicates an anagram.
'is'+'bust'='isbust'
'isbust' with letters rearranged gives 'ubsist'.
's'+'ubsist'='SUBSIST'
'to' is the link.
(Other definitions for subsist that I've seen before include "Support oneself at minimal level" , "Live at a minimal level" , "Have the necessities for life" , "Get by at minimum level" , "Survive at minimal level" .)