Made up for United having dismal start (6)
I believe the answer is:
atoned
'made up for' is the definition.
(I've seen this in another clue)
'united having dismal start' is the wordplay.
'united' becomes 'atone' ('at one' can mean connected or united).
'having' says to put letters next to each other.
'start' suggests taking the first letters.
The first letter of 'dismal' is 'd'.
'atone'+'d'='ATONED'
(Other definitions for atoned that I've seen before include "Expiated" , "Made amends" , "Made up for past offence" , "Made up for offence or sin" , "Made redress" .)