Demonstrated method to open tin (5)
I believe the answer is:
shown
'demonstrated' is the definition.
(I've seen this before)
'method to open tin' is the wordplay.
'method' becomes 'how' (I've seen this in another clue).
'to open' means one lot of letters goes inside another (some letters create an opening in a word).
'tin' becomes 'Sn' (Sn is the chemical symbol for tin).
'how' placed within 'sn' is 'SHOWN'.
(Other definitions for shown that I've seen before include "Displayed" , "Explained" , "Demonstrated" , "Exhibited" .)