Act as a preliminary for a unionist party statement finally being accepted by Varadkar (4,2,2)
I believe the answer is:
lead up to
'act as a preliminary' is the definition.
'a unionist party statement finally being accepted by varadkar' is the wordplay.
'unionist party' becomes 'dup' (Democratic Unionist Party).
'finally' suggests the final letters.
'being accepted by' means one lot of letters goes inside another.
'varadkar' becomes 'leo' (Leo Varadkar).
The final letter of 'statement' is 't'.
'a'+'dup'+'t'='adupt'
'adupt' inserted within 'leo' is 'LEAD UP TO'.
'for' is the link.
(Other definitions for lead up to that I've seen before include "Gradually approach" , "Serve as preparation, or direct conversation towards" , "Anticipate" , "prepare in stages" , "Serve as introduction to or preparation for" .)