Spurs ground is lit, letting in header from Man United (7)
I believe the answer is:
stimuli
'spurs' is the definition.
(plural of stimulus, a spur or cause of something)
'ground is lit letting in header from man united' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'letting in' means one lot of letters goes inside another.
'header from' suggests taking the first letters.
The initial letters of 'man united' is 'mu'.
'is'+'lit'='islit'
'islit' is an anagram of 'stili'.
'stili' going around 'mu' is 'STIMULI'.
(Other definitions for stimuli that I've seen before include "Things that produce a response" , "they provoked reaction" , "Reaction provokers" , "Things promoting activity" , "Incentives, spurs" .)