Sign off please when the leader has departed (6)
I believe the answer is:
ratify
'sign off' is the definition.
(both can mean to approve)
'please when the leader has departed' is the wordplay.
'please' becomes 'gratify' (I've seen this before).
'when the leader has departed' means to remove the first letter.
'gratify' with its initial letter taken away is 'RATIFY'.
(Other definitions for ratify that I've seen before include "Sanction" , "Formally confirm" , "Approve and sign" , "Give validity to" , "Authenticate" .)