Infant's ailment (5)
I believe the answer is:
'infant's ailment' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for colic that I've seen before include "A severe tummy ache" , "Intestinal pain" , "Severe stomach pain" , "Acute abdominal pain" , "unpleasant gut feeling" .)