Tin containing first of yellow colour (4)
I believe the answer is:
cyan
'colour' is the definition.
(greenish blue colour)
'tin containing first of yellow' is the wordplay.
'tin' becomes 'can' (synonyms).
'containing' indicates putting letters inside.
'first of yellow' becomes 'y' (1st letter of 'yellow').
'can' placed around 'y' is 'CYAN'.
(Other definitions for cyan that I've seen before include "Greenish blue" , "shade of blue" , "Bluish-green" , "Blue-green colour" , "Blue hue" .)