Everything from Gatti at last seen in a French poem (8)
I believe the answer is:
universe
'everything' is the definition.
(everything in the universe)
'gatti at last seen in a french poem' is the wordplay.
'at last' indicates one should take the final letters.
'seen in' is an insertion indicator.
'a french' becomes 'un' ('a' in French).
'poem' becomes 'verse' (verse is a kind of poem).
The final letter of 'gatti' is 'i'.
'un'+'verse'='unverse'
'i' placed into 'unverse' is 'UNIVERSE'.
'from' acts as a link.
(Other definitions for universe that I've seen before include "Cosmos" , "Edible crustacean" , "The earth and all the heavenly bodies" , "mind-boggling concept" , "Everything, really" .)