One may be opposed to request (5)
I believe the answer is:
thumb
'request' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'one may be opposed' is the wordplay.
I cannot really see how this works, but
'be' could be 'hum' (humming is a kind of being) and 'hum' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for thumb that I've seen before include "Quickly turn pages" , "Attempt to obtain a lift" , "Try to get a lift" , "Hitch ride" , "One opposed to other manual workers" .)