Three-page note amended by Yankee offering medical treatment (4,7)
I believe the answer is:
gene therapy
'treatment' is the definition.
(I know that gene therapy is a type of treatment)
'three-page note amended by yankee offering medical' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'three' is 'ether' which is within the answer.
'page' could be 'p' and 'p' is found within the answer.
'note' could be 'n' (abbreviation) and 'n' is located in the answer.
'yankee' could be 'y' (phonetic alphabet: alpha, bravo, charlie etc.) and 'y' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for gene therapy that I've seen is " Treatment applied at cellular level".)