Key passage denied opening note (4)
I believe the answer is:
isle
'key' is the definition.
(I've seen this before)
'passage denied opening note' is the wordplay.
I cannot really see how this works, but
'note' could be 'e' (musical note) and 'e' is found within the answer.
The remaining letters 'isl' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for isle that I've seen before include "Skye, for example" , "Hawaii, perhaps" , "Detached place" , "Perspire" , "Man, for one" .)