One who changes sides, finding fault with common soldiers (8)
I believe the answer is:
defector
'one who changes sides' is the definition.
'fault with common soldiers' is the wordplay.
'fault' becomes 'defect' ('defect' can be a synonym of 'fault').
'with' says to put letters next to each other.
'common soldiers' becomes 'or' (military abbreviation for 'other ranks').
'defect'+'or'='DEFECTOR'
'finding' is the link.
(Other definitions for defector that I've seen before include "E.g. Philby" , "One joining an opposing cause" , "Deserter changing sides" , "subject of court martial?" , "Person who deserts and joins the enemy" .)