Cat and dog ultimately in row (5)
I believe the answer is:
tiger
'cat' is the definition.
(tiger is a kind of cat)
'dog ultimately in row' is the wordplay.
'ultimately' suggests the final letters.
'in' means one lot of letters goes inside another.
'row' becomes 'tier' (tier is a kind of row**).
The last letter of 'dog' is 'g'.
'g' going inside 'tier' is 'TIGER'.
'and' is the link.
(Other definitions for tiger that I've seen before include "''... ... burning bright, in the forests of the night'' (Blake)" , "___ Woods, golfer" , "Striped carnivore" , "Large wild cat" , "Shere Khan's type" .)