Sharp, like a metal box (5)
I believe the answer is:
canny
'sharp' is the definition.
(I've seen this in another clue)
'a metal box' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
'metal' could be 'y' (Y is an example) and 'y' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'like' is the 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" .)