Stops sit-in organised around school period (9)
I believe the answer is:
intermits
'stops' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how they can define each other.
'sit-in organised around school period' is the wordplay.
'organised' indicates an anagram.
'around' is an insertion indicator.
'school period' becomes 'term' (I've seen this before).
'sit'+'in'='sitin'
'sitin' anagrammed gives 'inits'.
'inits' going around 'term' is 'INTERMITS'.
Can you help me to learn more?
(Another definition for intermits that I've seen is " Pauses".)