Row in stadium about good competitor (5)
I believe the answer is:
tiger
'competitor' is the definition.
(I have seen 'Formidable competitor' mean 'tiger' so perhaps 'competitor' could also mean 'tiger')
'row in stadium about good' is the wordplay.
'row in stadium' becomes 'tier' (row of seats in a stadium).
'about' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'tier' enclosing 'g' is 'TIGER'.
(Other definitions for tiger that I've seen before include "dynamic economy" , "Dead Celtic feline" , "Ferocious person" , "See 13" , "fierce animal" .)