Colour seen in metallic rims once (7)
I believe the answer is:
crimson
'colour' is the definition.
(crimson is a kind of colour)
'in metallic rims once' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CRIMSON' is hidden in the letters of 'metallic rims once'.
'seen' acts as a link.
(Other definitions for crimson that I've seen before include "Blood-red" , "Flush" , "Become red-faced" , "Deep red colour" , "Shade of red" .)