Repeatedly discovered fine string so essential (9)
I believe the answer is:
intrinsic
'essential' is the definition.
(associated in meaning)
'repeatedly discovered fine string so' is the wordplay.
'repeatedly discovered' indicates the central letters (two layers of letters are removed).
'string' becomes 'trin' (I can't explain this - if you can you should give a lot more credence to this answer).
'so' becomes 'sic' ('so' in Latin).
The middle letters of 'fine' are 'in'.
'in'+'trin'+'sic'='INTRINSIC'
Can you help me to learn more?
(Other definitions for intrinsic that I've seen before include "Belonging as an inherent and essential part" , "Naturally belonging" , "Belonging by its very nature" , "Natural" , "Genuine" .)