Deny withdrawing group of secret agents (6)
I believe the answer is:
negate
'deny' is the definition.
(I've seen this in other clues)
'withdrawing group of secret agents' is the wordplay.
'withdrawing' shows that the letters should be reversed in order.
'group of' says the answer is hidden in the clue (group of letters from the clue).
'etagen' can be found hidden inside 'secret agents'.
'etagen' written backwards gives 'NEGATE'.
(Other definitions for negate that I've seen before include "Cancel; invalidate" , "Nullify - deny the existence of" , "Countermand" , "Show to be false" , "disallow!" .)