Have expertise -- able to tell a scallion from a shallot? (4,4,6)
I believe the answer is:
know ones onions
'have expertise able to tell' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'a scallion from a shallot?' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'one' and 'one' is located in the answer.
'from' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'shallot?' could be 'onion' (shallot is a kind of onion) and 'onion' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for know ones onions that I've seen before include "Be very intelligent and well-informed" , "Read tear-jerkers" , "appear an expert?" , "Be an authority" , "Be very well informed" .)