I’m in surrogate ruler’s section of army (8)
I believe the answer is:
regiment
'section of army' is the definition.
'i'm in surrogate ruler's' is the wordplay.
'in' is an insertion indicator.
'surrogate ruler' becomes 'regent' (someone who rules on behalf of the monarch).
'im' put inside 'regent' is 'REGIMENT'.
(Other definitions for regiment that I've seen before include "Military group" , "The Grenadier Guards, for example" , "Military unit" , "Army group" , "Organise; army unit" .)