How pale Shane turns! (5)
I believe the answer is:
ashen
'how pale' is the definition.
'ashen' can be an answer for 'pale' (synonyms). I'm not certain of the 'how' bit.
'shane turns' is the wordplay.
'turns' indicates an anagram.
'shane' is an anagram of 'ASHEN'.
Can you help me to learn more?
(Other definitions for ashen that I've seen before include "showing signs of anaemia" , "Visibly shocked" , "White-faced" , "may be pale with shock" , "'Very pale, from shock say (5)'" .)