Insect eating mollusc once (8)
I believe the answer is:
formerly
'once' is the definition.
('formerly' can be a synonym of 'once')
'insect eating mollusc' is the wordplay.
'insect' becomes 'fly' (fly is a kind of insect).
'eating' is an insertion indicator.
'mollusc' becomes 'ormer' (ormer is a kind of mollusc).
'fly' placed around 'ormer' is 'FORMERLY'.
(Other definitions for formerly that I've seen before include "Previously, before this time" , "Hitherto" , "In the past" , "At one time" .)