I am interrupting fussily obstinate creature (6)
I believe the answer is:
animal
'creature' is the definition.
(I know that creature can be written as animal)
'i am interrupting fussily obstinate' is the wordplay.
'i am' becomes 'I'm' (contraction).
'interrupting' means one lot of letters goes inside another.
'fussily obstinate' becomes 'anal' (anal can mean overly fussy or orderly).
'im' inserted inside 'anal' is 'ANIMAL'.
(Other definitions for animal that I've seen before include "Circus performer, maybe" , "' - - Farm', Orwell" , "Any organism capable of voluntary movement" , "Brutish sort" , "Such as 13 ac" .)