Drink fell, missing exterior jacket flap (5)
I believe the answer is:
lapel
'jacket flap' is the definition.
(I've seen this in another clue)
'drink fell missing exterior' is the wordplay.
'drink' becomes 'lap' (to lap is to drink up).
'fell missing exterior' becomes 'el' (I can't justify this - if you can you should give a lot more credence to this answer).
'lap'+'el'='LAPEL'
Can you help me to learn more?
(Other definitions for lapel that I've seen before include "Jacket fold" , "Fold of coat" , "Part of a folded back collar" , "Lower extension of collar on jacket or coat" , "Continuation of a coat collar" .)