One must be fit to have it (8)
I believe the answer is:
aptitude
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'fit to have it' is the wordplay.
'fit' becomes 'apt' (apt means appropriate or fitting).
'to' becomes 'ude' (I can't explain this - if you can you should give a lot more credence to this answer).
'have' is an insertion indicator.
'apt'+'ude'='aptude'
'aptude' placed around 'it' is 'APTITUDE'.
'must be' acts as a link.
Can you help me to learn more?
(Other definitions for aptitude that I've seen before include "Natural gift" , "Bent" , "Update it, having natural skill" , "fitness" , "Update it, having a relevant talent" .)