Fatty is a dope unfortunately (7)
I believe the answer is:
adipose
'fatty' is the definition.
(synonyms)
'is a dope unfortunately' is the wordplay.
'unfortunately' indicates an anagram (a bad or unfortunate spelling).
'is'+'a'+'dope'='isadope'
'isadope' anagrammed gives 'ADIPOSE'.
(Other definitions for adipose that I've seen before include "Fat-storing (tissue)" , "Like a sort of fat" , "I soaped up with fatty kind of tissue" , "Is a dope fat?" , "Composed of animal fat" .)