Worry comedian Phil's doing a head-stand (4)
I believe the answer is:
gnaw
'worry' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for gnaw that I've seen before include "Prey on one's mind" , "What rodents typically do" , "Chew steadily at" , "Chew away at" , "Bite persistently, grind" .)