Reason soldier in company left in retreat (5)
I believe the answer is:
logic
'reason' is the definition.
(I've seen this before)
'soldier in company left in retreat' is the wordplay.
'soldier' becomes 'gi' (government issue or general issue).
'in' indicates putting letters inside.
'company' becomes 'co' (abbreviation).
'left' becomes 'l' (common abbreviation**).
'in retreat' is a reversal indicator.
'co'+'l'='col'
'col' written backwards gives 'loc'.
'gi' put within 'loc' is 'LOGIC'.
(Other definitions for logic that I've seen before include "using premises" , "Good reason" , "System of reasoning" , "Science of correct reasoning" , "Rationale ?" .)