Writhe, with convulsions, becoming increasingly pale (6)
I believe the answer is:
whiter
'increasingly pale' is the definition.
(more pale or white)
'writhe with convulsions' is the wordplay.
'with convulsions' is an anagram indicator.
'writhe' is an anagram of 'WHITER'.
'becoming' is the link.
(Other definitions for whiter that I've seen before include "''A ... Shade of Pale'' (Procul Harum)" , "A - - Shade of Pale (1960s song)" , "More like the snow" .)