Somewhat unbearable horse (4)
I believe the answer is:
arab
'horse' is the definition.
(Arab is a type of horse)
'somewhat unbearable' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'ARAB' can be found hidden inside 'unbearable'.
(Other definitions for arab that I've seen before include "Mount" , "Person from the Middle East, say" , "One from Saudi" , "Someone from the Middle East or North Africa, perhaps" , "Small riding horse" .)