Beast is first to get involved in row (5)
I believe the answer is:
tiger
'beast' is the definition.
(type of animal)
'first to get involved in row' is the wordplay.
'first to' indicates taking the first letters.
'involved in' means one lot of letters goes inside another.
'row' becomes 'tier' (a tier is a row of things).
The initial letter of 'get' is 'g'.
'g' placed inside 'tier' is 'TIGER'.
'is' acts as a link.
(Other definitions for tiger that I've seen before include "Predator" , "Carnivorous large cat" , "Cat; -- Woods" , "Dynamo" , "Endangered carnivore" .)