Reason officer turns up to grab soldier (5)
I believe the answer is:
logic
'reason' is the definition.
(I've seen this before)
'officer turns up to grab soldier' is the wordplay.
'officer' becomes 'col' (I've seen this before).
'turns up' is a reversal indicator.
'to grab' means one lot of letters goes inside another.
'soldier' becomes 'gi' (government issue or general issue).
'col' backwards is 'loc'.
'loc' going around 'gi' is 'LOGIC'.
(Other definitions for logic that I've seen before include "Sound sense" , "this makes sense" , "Sound reasoning" , "Rigorous reasoning" , "Science of correct reasoning" .)