Mischievous child allowed, it's understood (8)
I believe the answer is:
implicit
'understood' is the definition.
(similar in meaning)
'mischievous child allowed' is the wordplay.
'mischievous child' becomes 'imp' (imp can mean a misbehaving child).
'allowed' becomes 'licit' (I've seen this in another clue).
'imp'+'licit'='IMPLICIT'
'it's' acts as a link.
(Other definitions for implicit that I've seen before include "Assumed, not spelled out" , "Not actually stated" , "Inherent but not made clear" , "Suggested though not directly expressed" , "present, but not visible" .)