Agent is sure about informer having to go back in (9)
I believe the answer is:
'agent' is the definition.
(I know that surrogate is a type of agent)
'sure about informer having to go back in' is the wordplay.
'about' is an insertion indicator.
'informer' becomes 'rat' (synonyms).
'having' is an insertion indicator.
'to go back in' becomes 'og' (I can't justify this - if you can you should believe this answer much more).
'sure' enclosing 'rat' is 'surrate'.
'surrate' enclosing 'og' is 'SURROGATE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for surrogate that I've seen before include "Argues about rot with substitute or deputy" , "Deputising substitute" , "One standing in" , "stand-in" , "Substitute fulfilling the role of another" .)