United, within limits, thrashing Spurs (7)
I believe the answer is:
stimuli
'spurs' is the definition.
(plural of stimulus, a spur or cause of something)
'united within limits thrashing' is the wordplay.
'united' becomes 'U' (abbreviation).
'within' is an insertion indicator.
'thrashing' indicates anagramming the letters (I've seen this in other clues).
'limits' is an anagram of 'stimli'.
'u' inserted within 'stimli' is 'STIMULI'.
(Other definitions for stimuli that I've seen before include "Things that provoke reactions" , "Promoters of activity" , "Incentives, spurs" , "Rousing influences" , "Things that produce a response" .)