Terribly idle, inwardly you flap (6)
I believe the answer is:
eyelid
'flap' is the definition.
(eyelid is a kind of flap)
'terribly idle inwardly you' is the wordplay.
'terribly' indicates anagramming the letters.
'inwardly' is an insertion indicator (I've seen this in other clues).
'you' becomes 'ye' (historical version of 'you').
'idle' anagrammed gives 'elid'.
'elid' placed around 'ye' is 'EYELID'.
(Other definitions for eyelid that I've seen before include "Facial feature" , "Natural optical protector" , "Facial shutter" , "Protective cover" , "Where stye may occur" .)