Friend married on the rebound, causing a flap (6)
I believe the answer is:
dewlap
'flap' is the definition.
(flap of skin on an animal's neck)
'friend married on the rebound' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'married' becomes 'wed' (wed can mean married).
'on the rebound' says the letters should be written backwards (to rebound is to go backwards).
'pal'+'wed'='palwed'
'palwed' backwards is 'DEWLAP'.
'causing a' is the link.
(Other definitions for dewlap that I've seen before include "Fold of flesh hanging below chin" , "Some hide" , "one may hang by the neck" , "Pendulous skin under throat" , "Hanging fold of loose skin on an elderly person's neck" .)