Aftereffect of hair perm (5,4)
I believe the answer is:
shock wave
'aftereffect' is the definition.
'hair perm' is the wordplay.
'hair' becomes 'shock' (I've seen this before).
'perm' becomes 'wave' (wavy hair style).
'shock'+'wave'='SHOCK WAVE'
'of' is the link.
(Other definitions for shock wave that I've seen before include "It's generated by supersonic movement" , "Effect of an explosion" .)