Wild animal beginning to growl in row (5)
I believe the answer is:
tiger
'wild animal' is the definition.
'beginning to growl in row' is the wordplay.
'beginning to' says to take the initial letters.
'in' means one lot of letters goes inside another.
'row' becomes 'tier' (tier is a kind of row).
The initial letter of 'growl' is 'g'.
'g' placed within 'tier' is 'TIGER'.
(Other definitions for tiger that I've seen before include "The Celtic one has died" , "Shere Khan's type" , "Large wildcat poetically burning bright" , "Striped carnivore" , "Formidable competitor" .)