Spurs game I must lampoon, ironically at first (7)
'spurs' is the definition.
(plural of stimulus, a spur or cause of something)
'game i must lampoon ironically at first' is the wordplay.
'game' indicates an anagram (solving anagrams is a game).
'at first' indicates taking the first letters.
The initial letters of 'lampoon ironically' is 'li'.
'imust' anagrammed gives 'stimu'.
(Other definitions for stimuli that I've seen before include "Incentives, spurs", "Things that cause a response", "Actions producing a response", "Activity inducers", "they provoked reaction".)