Fancy, but ultimately modest (7)
I believe the answer is:
thought
'fancy' is the definition.
(thesaurus)
'but ultimately modest' is the wordplay.
'but' becomes 'though' (I've seen this before).
'ultimately' says to take the final letters.
The last letter of 'modest' is 't'.
'though'+'t'='THOUGHT'
(Other definitions for thought that I've seen before include "Cogitation" , "Study" , "Thinking process" , "Deliberated" , "Performed brain operations" .)