New trouble to fix (4)
I believe the answer is:
nail
'fix' is the definition.
(thesaurus)
'new trouble' is the wordplay.
'new' becomes 'n'.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
'n'+'ail'='NAIL'
'to' acts as a link.
(Other definitions for nail that I've seen before include "A small steel wood pin" , "Horny covering of the digits" , "for the carpenter?" , "Hit this on the head to be dead right" , "Amall metal spike" .)