Cheer AA after breakdown causing pain (7)
I believe the answer is:
earache
'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.
'cheer'+'aa'='cheeraa'
'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" .)