Mollusc trapped by insect once (8)
I believe the answer is:
formerly
'once' is the definition.
(I know that once can be written as formerly)
'mollusc trapped by insect' is the wordplay.
'mollusc' becomes 'ormer' (ormer is a kind of mollusc).
'trapped by' means one lot of letters goes inside another.
'insect' becomes 'fly' (fly is a kind of insect).
'ormer' going inside 'fly' is 'FORMERLY'.
(Other definitions for formerly that I've seen before include "Hitherto" , "At one time" , "In the past" , "Previously, before this time" .)