Pain hard in each competition (7)
I believe the answer is:
earache
'pain' is the definition.
(I know that earache is a type of ache)
'hard in each competition' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'in' indicates putting letters inside.
'each' becomes 'ea' (abbreviation).
'competition' becomes 'race' (race is a kind of competition).
'ea'+'race'='earace'
'h' put within 'earace' is 'EARACHE'.
(Other definitions for earache that I've seen before include "a problem for some listeners" , "Pain for listener" , "otitis" , "Aural discomfort" , "Lughole pain" .)