Bizarrely, our tree sloth is to be in control (4,3,5)
I believe the answer is:
rule the roost
'is to be in control' is the definition.
(I've seen this before)
'bizarrely our tree sloth' is the wordplay.
'bizarrely' indicates anagramming the letters.
'our'+'tree'+'sloth'='ourtreesloth'
'ourtreesloth' anagrammed gives 'RULE THE ROOST'.
(Other definitions for rule the roost that I've seen before include "Be in control" , "Be top hen" , "Predominate" , "Dominate (as chief hen?)" , "Rower" .)