Fatty Arbuckle's debut surprisingly poised (7)
I believe the answer is:
adipose
'fatty' is the definition.
(synonyms)
'arbuckle's debut surprisingly poised' is the wordplay.
'debut' suggests taking the first letters.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
The first letter of 'arbuckles' is 'a'.
'poised' is an anagram of 'dipose'.
'a'+'dipose'='ADIPOSE'
(Other definitions for adipose that I've seen before include "Fat-storing (tissue)" , "I soaped up with fatty kind of tissue" , "Containing fat" , "Relating to fatty tissue" , "Like fatty tissue" .)