Idea, albeit ultimately deficient (7)
I believe the answer is:
thought
'idea' is the definition.
(synonyms)
'albeit ultimately deficient' is the wordplay.
'albeit' becomes 'though'.
'ultimately' says to take the final letters.
The last letter of 'deficient' is 't'.
'though'+'t'='THOUGHT'
(Other definitions for thought that I've seen before include "Performed brain operations" , "Orwell's sort of police" , "Deliberated" , "Fancy" , "Musing" .)