Missing the odd school disco is a pain (5)
I believe the answer is:
colic
'pain' is the definition.
(colic is a kind of pain)
'missing the odd school disco' is the wordplay.
'missing the odd' indicates alternate letters (letters in odd-numbered positions removed).
'school'+'disco'='schooldisco'
The alternating letters of 'schooldisco' are 'COLIC'.
'is a' is the link.
(Other definitions for colic that I've seen before include "suffering" , "Abdominal pain, especially in infants" , "Gastric pain" , "Abdominal pain spasms" , "Illness" .)