Keen to get to the heart of one's subject of enthusiasm (4)
I believe the answer is:
into
'keen' is the definition.
'to get to the heart of one's subject of enthusiasm' is the wordplay.
'to get' is an insertion indicator.
'to the heart of' means to look at the middle letters.
'one's' becomes 'un's' (northern English dialect form of 'one').
'subject of enthusiasm' becomes 'ito' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The centre of 'uns' is 'n'.
'n' inserted inside 'ito' is 'INTO'.
Can you help me to learn more?
(Other definitions for into that I've seen before include "To a point within - not I" , "Taking a lively and active interest in (colloq.)" , "Keen on (informal)" , "Knowledgeable about" , "Fascinated by" .)