Obscure allusion picked up, getting harder to pick up (10)
I believe the answer is:
diminuendo
'up' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'obscure allusion picked up getting harder to pick' is the wordplay.
I cannot really see how this works, but
'obscure' could be 'dim' (both can mean dark) and 'dim' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for diminuendo that I've seen before include "becoming quieter" , "volume is to be cut" , "Letting sound fade away" , "Decrease in loudness" , "losing force" .)