It's canny to keep biscuits in one (3)
I believe the answer is:
tin
'one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'it's canny to keep biscuits in' is the wordplay.
I cannot quite see how this works, but
'in' is present in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tin that I've seen before include "Element, symbol Sn" , "Silvery metallic element" , "roofing material" , "Kind of metal often used as food container" , "loaf of bread" .)