United brought in successful player who cleans up? (7)
I believe the answer is:
scourer
'who cleans up?' is the definition.
(someone who scours)
'united brought in successful player' is the wordplay.
'united' becomes 'u'.
'brought in' indicates putting letters inside.
'successful player' becomes 'scorer' (eg someone who scores goals).
'u' placed within 'scorer' is 'SCOURER'.
(Other definitions for scourer that I've seen before include "one cleans up" , "I'll search high and low" , "Washing-up aid" , "Pot cleaner" , "Scout, perhaps" .)