Row about granny's first big cat (5)
I believe the answer is:
tiger
'big cat' is the definition.
(I know that tiger is a type of big cat)
'row about granny's first' is the wordplay.
'row' becomes 'tier' (a tier is a row of things).
'about' means one lot of letters goes inside another.
'granny's first' becomes 'g' (tst letter of 'granny').
'tier' enclosing 'g' is 'TIGER'.
(Other definitions for tiger that I've seen before include "fierce animal" , "dynamic economy" , "Striped carnivore" , "Ferocious cat" , "Beast - type of 1 across" .)