One who may dismiss envelope’s meaning (5,5)
I believe the answer is:
cover point
'one who' is the definition.
The definition suggests a singular noun which matches the answer.
'dismiss envelope's meaning' is the wordplay.
'dismiss envelope' becomes 'cover' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'meaning' becomes 'point' (point is a kind of meaning).
'cover'+'point'='COVER POINT'
'may' is the link.
Can you help me to learn more?
(Other definitions for cover point that I've seen before include "Fielder" , "Man in the field" .)