Expose a secret (4)
I believe the answer is:
tell
'expose a secret' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tell that I've seen before include "Count - recount" , "Inform - count - Swiss hero" , "... national hero" , "Rossini composed opera 'William ....'" , "Instruct" .)