Notice one set is fat (7)
I believe the answer is:
adipose
'is fat' is the definition.
(similar in meaning)
'notice one set' is the wordplay.
'notice' becomes 'ad' (both can mean an advertisement).
'one' becomes 'i' (Roman numeral).
'set' becomes 'pose' (posing is a kind of setting).
'ad'+'i'+'pose'='ADIPOSE'
(Other definitions for adipose that I've seen before include "Composed of animal fat" , "Of tissue, fatty in substance" , "Of animal, fat, fatty" , "Relating to fatty tissue" , "Of tissue storing fat" .)