Left-winger rejects first and second parties (7)

'parties' is the definition.
(social is a kind of party)

'left-winger rejects first and second' is the wordplay.
I cannot really see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is found within the answer.
'second' could be 's' (common abbreviation) and 's' is found in the answer.
The remaining letters 'ocas' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?