Reported old Premier joined party for drink (7)
I believe the answer is:
sidecar
'reported old premier joined party for drink' is the definition.
'sidecar' can be an answer for 'drink' (sidecar is a kind of drink). I'm not certain of the 'reported old premier joined party for' bit.
'party for drink' is the wordplay.
'party' becomes 'side' (eg a side or party in a contract).
'for drink' becomes 'car' (I can't justify this - if you can you should give a lot more credence to this answer).
'side'+'car'='SIDECAR'
Can you help me to learn more?
(Other definitions for sidecar that I've seen before include "Alcoholic drink" , "Passenger carrier attached to a motorcycle" , "Motorbike passenger attachment" , "Motorcycle accessory -- cocktail" , "Vehicle" .)