Policy of alliance oddly abandoned (4)
I believe the answer is:
line
'policy' is the definition.
(I've seen this before)
'alliance oddly abandoned' is the wordplay.
'oddly abandoned' means one should take alternating letters (letters in odd-numbered positions removed).
The alternate letters of 'alliance' are 'LINE'.
'of' is the link.
(Other definitions for line that I've seen before include "A length without breadth or thickness" , "An American queue" , "Some poetic feet; boundary, when drawn" , "Railway track; policy" , "classified information" .)