Soldier on exercises runs hard (9)
I believe the answer is:
persevere
'soldier on' is the definition.
(I've seen this before)
'exercises runs hard' is the wordplay.
'exercises' becomes 'pe' (Physical Education class).
'runs' becomes 'r' (cricket abbreviation).
'hard' becomes 'severe' (synonyms).
'pe'+'r'+'severe'='PERSEVERE'
(Other definitions for persevere that I've seen before include "Keep trying" , "Keep going" , "Continue despite difficulties" , "Go on in spite of setbacks" , "Remain" .)