File used occasionally to create vent (4)
I believe the answer is:
flue
'vent' is the definition.
(I've seen this in another clue)
'file used occasionally' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'file'+'used'='fileused'
The alternate letters of 'fileused' are 'FLUE'.
'to create' acts as a link.
(Other definitions for flue that I've seen before include "Chimney vent" , "Tube" , "Smoke-duct in a chimney" , "Air vent" , "Fuel (anag.)" .)