A bad-tempered over (6)
I believe the answer is:
across
'over' is the definition.
('across' can be a synonym of 'over')
'a bad-tempered' is the wordplay.
'bad-tempered' becomes 'cross' ('cross' can be a synonym of 'bad-tempered').
'a'+'cross'='ACROSS'
(Other definitions for across that I've seen before include "Clue with solution not going in the direction of this one!" , "Astride" , "written like this?" , "sort of clue?" , "ax" .)