Squirm like a man under summons? (6)
I believe the answer is:
writhe
'squirm' is the definition.
(synonyms)
'man under summons?' is the wordplay.
'man' becomes 'he' (he refers to a man).
'under' says to put letters next to each other (below, in a down clue).
'summons?' becomes 'writ' (legal summons).
'he' put after 'writ' is 'WRITHE'.
'like a' is the link.
(Other definitions for writhe that I've seen before include "Wriggle" , "feel acute embarrassment" , "worm" , "Squirm in pain" , "'Twist, squirm (6)'" .)