Sickness of sheep that is seen after fight (7)
I believe the answer is:
scrapie
'sickness of sheep' is the definition.
'that is seen after fight' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'seen after' says to put letters next to each other.
'fight' becomes 'scrap' (I've seen this before).
'ie' after 'scrap' is 'SCRAPIE'.
(Other definitions for scrapie that I've seen before include "Disease of sheep" , "beastly disease" , "sheep killer" , "It affects sheep" .)