Dynamic person good in row (5)
I believe the answer is:
tiger
'dynamic person' is the definition.
(tiger can mean an ambitious or dynamic person)
'good in row' is the wordplay.
'good' becomes 'g' (abbreviation).
'in' is an insertion indicator.
'row' becomes 'tier' (a tier is a row of things).
'g' placed into 'tier' is 'TIGER'.
(Other definitions for tiger that I've seen before include "Beast - type of 1 across" , "Formidable competitor" , "audacious person" , "he got his stripes" , "Large cat with black stripes" .)