Unlikely that mischievous child can ever wear a gown! (10)
I believe the answer is:
improbable
'unlikely' is the definition.
(I know that unlikely can be written as improbable)
'mischievous child can ever wear a gown' is the wordplay.
'mischievous child' becomes 'imp' (imp can mean a misbehaving child).
'can ever' becomes 'abl' (I can't justify this - if you can you should give a lot more credence to this answer).
'wear' is an insertion indicator (to wear something is to be covered up by it).
'a gown' becomes 'robe' ('robe' can be a synonym of 'gown').
'abl' going inside 'robe' is 'robable'.
'imp'+'robable'='IMPROBABLE'
'that' is the link.
Can you help me to learn more?
(Other definitions for improbable that I've seen before include "Highly unlikely" , "Far-fetched" , "Not likely" .)