Fix marathon's start and finish (4)
I believe the answer is:
mend
'fix' is the definition.
(synonyms)
'marathon's start and finish' is the wordplay.
'start' says to take the initial letters.
'and' means one lot of letters go next to another.
'finish' becomes 'end' (come to an end).
The initial letter of 'marathons' is 'm'.
'm'+'end'='MEND'
(Other definitions for mend that I've seen before include "'Restore, fix (4)'" , "Put right" , "Fix, repair" , "Heal" , "Make improvements" .)