Irregular glue ear? Dr will clean occasionally (9)
I believe the answer is:
guerrilla
'irregular' is the definition.
(synonyms)
'glue ear? dr will clean occasionally' is the wordplay.
'occasionally' indicates alternate letters.
'glue'+'ear'+'dr'+'will'+'clean'='glueeardrwillclean'
The alternating letters of 'glueeardrwillclean' are 'GUERRILLA'.
(Other definitions for guerrilla that I've seen before include "Irregular soldier" , "Member of an irregular armed force" , "Partisan" , "Irregular bandit" , "Irregular fighter" .)