Animal to remain in group (5,6)
I believe the answer is:
stick insect
'animal' is the definition.
(stick insect is a kind of animal)
'remain in group' is the wordplay.
'remain' becomes 'stick' (both can mean to stay).
'group' becomes 'sect' (I've seen this in other clues).
'stick'+'in'+'sect'='STICK-INSECT'
'to' is the link.
(Other definitions for stick insect that I've seen before include "Twig-like creature" , "Bug that's noted for camouflage" , "creature hard to spot in tree" , "Crawler" , "leaf creature?" .)