Staff used to celebrate once prime minister called election (7)
I believe the answer is:
maypole
'staff used to celebrate' is the definition.
(pole used on May Day)
'prime minister called election' is the wordplay.
'prime minister' becomes 'may' (I've seen this in other clues).
'called election' becomes 'pole' (I can't explain this - if you can you should give a lot more credence to this answer).
'may'+'pole'='MAYPOLE'
'once' is the link.
Can you help me to learn more?
(Other definitions for maypole that I've seen before include "Traditional dancing venue" , "Danced-around post" , "Decorated springtime post" , "at the centre of celebration?" , "In celebration, one danced round" .)