Squirm and turn whiter (6)
I believe the answer is:
writhe
'squirm' is the definition.
(synonyms)
'turn whiter' is the wordplay.
'turn' is an anagram indicator.
'whiter' is an anagram of 'WRITHE'.
'and' is the link.
(Other definitions for writhe that I've seen before include "Contort in pain" , "Painfully move" , "Twist as in pain" , "Move sinuously" , "'Twist, squirm (6)'" .)