Trick with needle to get swelling out (6)
I believe the answer is:
convex
'swelling out' is the definition.
(convex means curving outwards)
'trick with needle' is the wordplay.
'trick' becomes 'con' (a con is a fraud or trick).
'with' means one lot of letters go next to another.
'needle' becomes 'vex' (both can mean to annoy).
'con'+'vex'='CONVEX'
'to get' acts as a link.
(Other definitions for convex that I've seen before include "Curved like surface of a ball" , "Curved outwards" , "Curving outwards" , "Curving like surface of ball" , "Rounded on the outside" .)