It is, you realise, a question of skill (4-3)
I believe the answer is:
'skill' is the definition.
(I've seen this before)
'it is you realise a question' is the wordplay.
I cannot quite see how this works, but
'realise' could be 'know' (I've seen this before) and 'know' is located in the answer.
'question' could be 'how' (I've seen this in other clues) and 'how' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for know-how that I've seen before include "Technique, expertise" , "Ingenuity - aptitude - skill" , "Ingenuity - experience" , "Ingenuity - aptitude - knack" , "Practical knowledge or skill" .)