Fix up extremely remarkable couple (6)
I believe the answer is:
repair
'fix up' is the definition.
(thesaurus)
'extremely remarkable couple' is the wordplay.
'extremely' means to remove the middle letters.
'couple' becomes 'pair' ('pair' can be a synonym of 'couple').
'remarkable' with its centre removed is 're'.
're'+'pair'='REPAIR'
(Other definitions for repair that I've seen before include "Get back into good shape" , "Mend - go" , "Restore to working condition" , "Make one's way (to); fix" , "Make good again" .)