Extremely or partly clever? Somewhat (4,2)
I believe the answer is:
ever so
'extremely' is the definition.
(I've seen this before)
'partly clever? somewhat' is the wordplay.
'partly' indicates a hidden word.
'EVER SO' can be found hidden inside 'clever somewhat'.
'or' acts as a link.
(Other definitions for ever so that I've seen before include "really" , "Extremely" , "Intensifier" , "Greatly" , "Jolly" .)