Header from Rooney inspired by United great one assumes (7)
I believe the answer is:
usurper
'one assumes' is the definition.
(a usurper assumes someone else's position)
'header from rooney inspired by united great' is the wordplay.
'header from' suggests taking the first letters.
'inspired by' indicates putting letters inside (some letters breathed in or 'inspired' by others).
'united' becomes 'U' (abbreviation as in Man U for Manchester United).
'great' becomes 'super' (both can mean excellent).
The initial letter of 'rooney' is 'r'.
'u'+'super'='usuper'
'r' inserted into 'usuper' is 'USURPER'.
(Other definitions for usurper that I've seen before include "replacing person" , "Unlawful seizer of power" , "One supplanting (ruler) by force" , "One seizes power illegally" , "One seizing power illegally" .)