Flier to believe completely (7)
I believe the answer is:
swallow
This might be a double definition.
'flier' is the first definition.
(swallow is a kind of birdanimal)
'believe completely' is the second definition.
'swallow' can be an answer for 'believe' (to swallow something is to accept or believe it). I'm unsure of the 'completely' bit.
'to' is the link.
Can you help me to learn more?
(Other definitions for swallow that I've seen before include "Buy < and consume" , "Ingest - bird" , "flier" , "Migrating bird" , "See bird" .)