Flap in a stable door (3)
I believe the answer is:
tab
'flap' is the definition.
(I've seen this before)
'in a stable door' is the wordplay.
'in' says the answer is hidden in the clue.
'TAB' is hidden in the letters of 'stable door'.
(Other definitions for tab that I've seen before include "Bar tally; small flap" , "Running bar bill" , "Tag" , "The bill in a restaurant" , "Tally of orders in bar" .)