Sharp, like a metal box (5)
I believe the answer is:
canny
'sharp' is the definition.
(thesaurus)
'a metal box' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
'metal' could be 'y' (Y is an example) and 'y' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'like' acts as a link.
Can you help me to learn more?
(Other definitions for canny that I've seen before include "Shrewd, knowing" , "Shrewd like Scotsman" , "Fly" , "Shrewd, clever" , "Worldly-wise" .)