Clarify what is no longer easily understood (7)
I believe the answer is:
explain
'clarify' is the definition.
(I know that clarify is a more specific form of the action explain)
'no longer easily understood' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'easily understood' becomes 'plain' (I can't explain this - if you can you should believe this answer much more).
'ex'+'plain'='EXPLAIN'
'what is' is the link.
Can you help me to learn more?
(Other definitions for explain that I've seen before include "Make understandable" , "Elucidate - define" , "area affected by mountain formation?" , "Show the whys and wherefores" , "Make intelligible and account for" .)