Seizes day to commit to alliance (4,5)
I believe the answer is:
take sides
'commit to alliance' is the definition.
(to take sides is to make an alliance)
'seizes day' is the wordplay.
'seizes' becomes 'takes' (I've seen this in another clue).
'day' becomes 'ides' (ides is a kind of day).
'takes'+'ides'='TAKE SIDES'
'to' acts as a link.
(Other definitions for take sides that I've seen before include "Support one of parties in dispute" , "Support one group over another" , "choose who to support" , "Support one faction" , "Join the struggle" .)