Temper rising, perhaps Queen is pessimist (9)

'pessimist' is the definition.
(a doomsayer has a pessimistic attitude)

'temper rising perhaps queen' is the wordplay.
'temper' becomes 'mood' (synonyms).
'rising' says the letters should be written backwards (going up in a down clue).
'perhaps' becomes 'say' (both can mean 'for example').
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'mood' backwards is 'doom'.
'doom'+'say'+'er'='DOOMSAYER'

'is' acts as a link.