Writhe about and shrivel up (6)
I believe the answer is:
wither
'shrivel up' is the definition.
(I know that shrivel up can be written as wither)
'writhe about' is the wordplay.
'about' indicates anagramming the letters.
'writhe' with letters rearranged gives 'WITHER'.
'and' acts as a link.
(Other definitions for wither that I've seen before include "Shrink, shrivel up" , "Become dry and shrivel" , "Dry up, become shrivelled" , "Shrivel away" , "Lose moisture and die" .)