American dad left former partner Amy finally in fit of infuriation (8)
I believe the answer is:
apoplexy
'fit of infuriation' is the definition.
(I've seen this before)
'american dad left former partner amy finally' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'dad' becomes 'pop' (synonyms).
'left' becomes 'l' (common abbreviation).
'former partner' becomes 'ex' (an ex-partner).
'finally' indicates one should take the final letters.
The final letter of 'amy' is 'y'.
'a'+'pop'+'l'+'ex'+'y'='APOPLEXY'
'in' is the link.
(Other definitions for apoplexy that I've seen before include "man fit" , "Rage" , "Rush of blood to the head, or a stroke" , "Speechlessness caused by fury" , "Rupture of a blood vessel or extreme anger" .)