Squirm as you scribble out a letter (6)
I believe the answer is:
writhe
'squirm' is the definition.
(synonyms)
'you scribble out a letter' is the wordplay.
'you scribble' becomes 'write' (I can't justify this - if you can you should give a lot more credence to this answer).
'out' means one lot of letters goes inside another.
'a letter' becomes 'h'.
'write' going around 'h' is 'WRITHE'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for writhe that I've seen before include "Thrash" , "Squirm in pain" , "6, maybe in agony" , "Twist in agony" , "Painfully move" .)