First animal starting going out is with some long-legged, burrowing rodents (7)
I believe the answer is:
agoutis
'rodents' is the definition.
(I've seen this before)
'first animal starting going out is' is the wordplay.
'first animal' becomes 'a' (1st letter of 'animal').
'starting' indicates taking the first letters.
The first letter of 'going' is 'g'.
'a'+'g'+'out'+'is'='AGOUTIS'
'with some long-legged burrowing' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for agoutis that I've seen before include "Rodents" , "More than one animal" .)