Very old key in crushed tin can (7)
I believe the answer is:
ancient
'very old' is the definition.
(I've seen this before)
'key in crushed tin can' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'crushed' is an anagram indicator.
'tin'+'can'='tincan'
'tincan' is an anagram of 'ancint'.
'e' inserted inside 'ancint' is 'ANCIENT'.
(Other definitions for ancient that I've seen before include "Old salt in Coleridge's poem" , "very old boy?" , "From the distant past" , "Nice tan (anag.)" , "Archaic" .)