Intermittent menstrual colic lasts (8)
I believe the answer is:
periodic
'intermittent' is the definition.
(similar in meaning)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for periodic that I've seen before include "At fixed intervals" , "Occurring or appearing at intervals" , "every now and then" , "Occasional" , "Happening at intervals" .)