Legal document getting that chap to squirm (6)
I believe the answer is:
writhe
'squirm' is the definition.
(synonyms)
'legal document getting that chap' is the wordplay.
'legal document' becomes 'writ' (writ is a kind of legal document).
'getting' is a charade indicator (letters next to each other).
'that chap' becomes 'he'.
'writ'+'he'='WRITHE'
'to' is the link.
(Other definitions for writhe that I've seen before include "Squirm in pain" , "Struggle" , "Twist as in pain" , "feel acute embarrassment" , "Wriggle" .)