Snoop, occasionally, grasps that it's forbidden, but implies they can do what they like (2-5-6)
I believe the answer is:
no-holds-barred
'implies they can do what they like' is the definition.
The definition suggests an adverb but the answer is not.
'snoop occasionally grasps that it's forbidden but' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'grasps' becomes 'holds' ('hold' can be a synonym of 'grasp').
'that it's forbidden but' becomes 'barred' (I am not sure about this - if you are sure you should believe this answer much more).
The alternate letters of 'snoop' are 'no'.
'no'+'holds'+'barred'='NO-HOLDS-BARRED'
Can you help me to learn more?
(Other definitions for no-holds-barred that I've seen before include "Not observing any rules or regulations" , "Unrestrained" , "Free from restrictions" , "Not observing the rules of fair play" , "Ignoring the rules of fair play" .)