Mark is bad-tempered (5)
I believe the answer is:
cross
I believe this clue is a double definition.
'mark' is the first definition.
(I know that mark can be written as cross)
'bad-tempered' is the second definition.
(synonyms)
'is' acts as a link.
(Other definitions for cross that I've seen before include "Short punch in boxing" , "severe trial" , "frustrated?" , "means of execution" , "Peeved" .)