Manchester United in this matter is synonymous with the health of The Roses (6)
I believe the answer is:
manure
'manchester united in this matter is synonymous with the health of the roses' is the definition.
'manure' can be an answer for 'matter' (manure is a kind of matter). I am unsure of the remainder of the definition.
'manchester united in this matter' is the wordplay.
'manchester united' becomes 'manu' (Man U is a common nickname for the team).
'in this' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'matter' becomes 'e' (E is an example).
'manu'+'r'+'e'='MANURE'
Can you help me to learn more?
(Other definitions for manure that I've seen before include "Natural animal fertiliser" , "Dung as fertiliser" , "Crap" , "Animal excreta as fertiliser" , "Muck" .)