Chicken given supplements ultimately displays signs of ageing (7)
I believe the answer is:
yellows
'displays signs of ageing' is the definition.
'chicken given supplements ultimately' is the wordplay.
'chicken' becomes 'yellow' (as in cowardly).
'given' means one lot of letters go next to another.
'ultimately' says to take the final letters.
The last letter of 'supplements' is 's'.
'yellow'+'s'='YELLOWS'
(Other definitions for yellows that I've seen before include "shows age" , "shades of autumn?" , "Plant disease" , "Primary colours" , "Turns lurid, perhaps" .)