Tin containing unknown colour (4)
I believe the answer is:
cyan
'colour' is the definition.
(greenish blue colour)
'tin containing unknown' is the wordplay.
'tin' becomes 'can' (synonyms).
'containing' is an insertion indicator.
'unknown' becomes 'y' (algebra).
'can' enclosing 'y' is 'CYAN'.
(Other definitions for cyan that I've seen before include "Blue-green colour" , "Greenish blue" , "Printer's colour" , "Printer's blue" , "Greenish-blue colour complementary to red" .)