Post Office protector is directly opposed (5)
I believe the answer is:
polar
'directly opposed' is the definition.
(I've seen this before)
'post office protector' is the wordplay.
'post office' becomes 'po' (abbreviation).
'protector' becomes 'lar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'po'+'lar'='POLAR'
'is' is the link.
Can you help me to learn more?
(Other definitions for polar that I've seen before include "extremes of 3 or 26 [NORTH OR SOUTH]" , "Type of bear" , "Diametrically opposed -- very cold" , "From one end of the Earth or the other" , "Relating to icy regions" .)