Spurs United to extraordinary limits (7)
I believe the answer is:
stimuli
'spurs' is the definition.
(plural of stimulus, a spur or cause of something)
'united to extraordinary limits' is the wordplay.
I cannot really see how this works, but
'united' could be 'u' (as in Man U, English football team) and 'u' is found within the answer.
an anagram of 'limits' is 'stimli' which is within the left over letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stimuli that I've seen before include "Activity inducers" , "Things that cause a response" , "Things that produce a response" , "Incentives to action" , "Incentives, spurs" .)