Break a rule about many an animal (5)
I believe the answer is:
lemur
'an animal' is the definition.
(lemur is a kind of animal)
'break a rule about many' is the wordplay.
'break' indicates an anagram.
'about' is an insertion indicator.
'many' becomes 'm' (Roman numeral for a thousand).
'rule' with letters rearranged gives 'leur'.
'leur' placed around 'm' is 'LEMUR'.
(Other definitions for lemur that I've seen before include "Monkey-like creature of Madagascar" , "A primate; sounds like a S. Am. city" , "Monkey-like animal of Madagascar" , "Animal with foxy face and long furry tail" , "Malagasy primate" .)