Protection from one small measure with agreement (8)
I believe the answer is:
immunity
'protection from one small' is the definition.
'immunity' can be an answer for 'protection' (thesaurus). I am not sure about the 'from one small' bit.
'measure with agreement' is the wordplay.
'measure with' becomes 'imm' (I can't justify this - if you can you should believe this answer much more).
'agreement' becomes 'unity' (I've seen this in another clue).
'imm'+'unity'='IMMUNITY'
Can you help me to learn more?
(Other definitions for immunity that I've seen before include "Inability to catch a cold" , "legal protection?" , "Exemption; resistance" , "Protection or exemption from something" , "Exemption (from, eg, a penalty)" .)