Endeavour to fool one younger (5)
I believe the answer is:
'endeavour' is the definition.
(I know that endeavour is a more specific form of the action assay)
'fool one younger' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'one' becomes 'a' (a thing is one thing).
'younger' becomes 'y' (this might be a standard abbreviation which I don't know about).
'to' is the link.
Can you help me to learn more?
(Other definitions for assay that I've seen before include "Test metal for quality" , "Test of substance" , "Put to the proof" , "Test the purity of metals" , "Test gold" .)