Cheer AA after breakdown causing pain (7)
I believe the answer is:
'pain' is the definition.
(I know that earache is a type of ache)
'cheer aa after breakdown' is the wordplay.
'after' means one lot of letters go next to another.
'breakdown' indicates anagramming the letters.
'cheeraa' with letters rearranged gives 'EARACHE'.
'causing' acts as a link.
(Other definitions for earache that I've seen before include "Form of torture" , "Pain in aural organ" , "Pain in an aural appendage" , "pain (from hairdryer treatment?)" , "sort of pain" .)