Insurance agent's final secret (6)
I believe the answer is:
'secret' is the definition.
('covert' can be similar in meaning to 'secret')
'insurance agent's final' is the wordplay.
'insurance' becomes 'cover' (I've seen this before).
'final' says to take the final letters.
The final letter of 'agent' is 't'.
(Other definitions for covert that I've seen before include "Not openly displayed" , "place for game" , "Concealed, like fox's lair" , "Area of undergrowth in which animals hide" , "in this secret place?" .)