Being eligible for full fare, note price downgrade (10)
I believe the answer is:
adulterate
'price downgrade' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
'being eligible for full fare note' is the wordplay.
I cannot really see how this works, but
'being' could be 'adult' (adult is a kind of being) and 'adult' is located in the answer.
'fare' could be 'rate' (I've seen this in another clue) and 'rate' is found within the answer.
'note' could be 'e' (musical note) and 'e' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for adulterate that I've seen before include "Contaminate" , "cut" , "Mix with a poorer substance" , "Doctor" , "Make impure by addition" .)