Set of animals not easily moved (6)
I believe the answer is:
stable
I believe this clue is a double definition.
'set of animals' is the first definition.
(group of horses)
'not easily moved' is the second definition.
(stable or fixed in place)
(Other definitions for stable that I've seen before include "Firm and dependable" , "Equine housing" , "far from bats?" , "Steady, unchanging" , "Permanent" .)