Turn out to be the last to join in eastern party (3,2)
I believe the answer is:
end up
'turn out' is the definition.
(I've seen this before)
'the last to join in eastern party' is the wordplay.
'the last to' says to take the final letters.
'in' is an insertion indicator.
'eastern' becomes 'e'.
'party' becomes 'dup' (I've seen this in other clues).
The last letter of 'join' is 'n'.
'e'+'dup'='edup'
'n' going within 'edup' is 'END UP'.
'to be' is the link.
(Other definitions for end up that I've seen before include "Eventually become" , "Ultimately become" , "Eventually find oneself" , "Finally do or become (something)" , "Arrive eventually" .)