Socialiser in 1009 — the Queen! (5)
I believe the answer is:
mixer
'socialiser' is the definition.
(I've seen this before)
'1009 the queen' is the wordplay.
'1009' becomes 'mix' (MIX is 1009 in Roman numerals ).
'the queen' becomes 'er' (Elizabeth Regina).
'mix'+'er'='MIXER'
'in' is the link.
(Other definitions for mixer that I've seen before include "Eg, tonic water" , "Soft drink; type of tap" , "Device for cement or in kitchen" , "Sociable mingler" , "Food processor" .)