A duck, for example, at no time turns this colour (4,5)
I believe the answer is:
nile green
'this colour' is the definition.
(I've seen this before)
'a duck for example at no time turns' is the wordplay.
'a duck' becomes 'nil' (I've seen this before).
'for example' becomes 'eg' (synonyms).
'at no time' becomes 'neer' (synonyms).
'turns' is a reversal indicator.
'neer' reversed gives 'reen'.
'nil'+'eg'+'reen'='NILE GREEN'
(Other definitions for nile green that I've seen before include "Pale shade" , "Pale colour" .)