The extent of one's knowledge? (6)
I believe the answer is:
degree
'the extent' is the definition.
(I've seen this before)
'one's knowledge?' is the wordplay.
I cannot really understand how this works, but
'knowledge?' could be 'ee' (EE is an example) and 'ee' is found in the answer.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for degree that I've seen before include "Further education award - extent" , "Bit" , "Position on a scale" , "Letters" , "Unit of measurement of angle" .)