Stinging insect squashed by most angry ruminant (10)
I believe the answer is:
wildebeest
'ruminant' is the definition.
(wildebeest is a kind of ruminant)
'stinging insect squashed by most angry' is the wordplay.
'stinging insect' becomes 'bee' (I've seen this before).
'squashed by' is an insertion indicator.
'most angry' becomes 'wildest' ('wild' can be a synonym of 'angry').
'bee' inserted inside 'wildest' is 'WILDEBEEST'.
(Other definitions for wildebeest that I've seen before include "Gnu" , "antelope" , "beast" , "large animal" , "game in Africa" .)