Well, Feds seized an invention (7)
I believe the answer is:
figment
'an invention' is the definition.
(I've seen this before)
'well feds seized' is the wordplay.
'well' becomes 'fit' (associated in meaning).
'feds' becomes 'gmen' (I've seen this in another clue).
'seized' is an insertion indicator.
'fit' enclosing 'gmen' is 'FIGMENT'.
(Other definitions for figment that I've seen before include "fantastic idea" , "Imaginary thing" , "Deception, sham" , "Fantastic notion" , "Thought" .)