Harden against some from Manchester United making a comeback (5)
I believe the answer is:
inure
'harden against' is the definition.
(I've seen this before)
'some from manchester united making a comeback' is the wordplay.
'some from' indicates a hidden word.
'making a comeback' says the letters should be written backwards.
'eruni' is hidden within 'manchester united'.
'eruni' in reverse letter order is 'INURE'.
(Other definitions for inure that I've seen before include "Steel" , "Temper, toughen" , "Harden against, make tough" , "Harden, habituate" , "harden attitude" .)