Small letter, more yellowy (8)
I believe the answer is:
sallower
'more yellowy' is the definition.
I can't tell whether this definition defines the answer.
'small letter' is the wordplay.
I cannot quite see how this works, but
'small' could be 'low' (synonyms) and 'low' is located in the answer.
'letter' could be 's' and 's' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for sallower that I've seen is " Looking comparatively pale".)