Keys disappear (4)
I believe the answer is:
fade
'disappear' is the definition.
(fading is a kind of disappearing)
'keys' is the wordplay.
I cannot quite understand how this works, but
'keys' could be 'de' (I've seen this in other clues) and 'de' is found in the answer.
The remaining letters 'fa' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for fade that I've seen before include "Gradually disappear from view" , "Die away" , "Lose brightness" , "Wither away" , "Diminish" .)