Each one comprehending what's easy? (6)
I believe the answer is:
apiece
'each' is the definition.
(synonyms)
'one comprehending what's easy?' is the wordplay.
'one' becomes 'ace' (synonyms).
'comprehending' means one lot of letters goes inside another (comprehend can mean include or contain).
'what's easy?' becomes 'pie' (I can't justify this - if you can you should believe this answer much more).
'ace' enclosing 'pie' is 'APIECE'.
Can you help me to learn more?
(Other definitions for apiece that I've seen before include "Per person" , "For or by each one of group" , "A head" , "for everyone" .)