Behold soldier caught reasoning (5)
I believe the answer is:
logic
'reasoning' is the definition.
(I've seen this before)
'behold soldier caught' is the wordplay.
'behold' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'soldier' becomes 'GI' (GI can mean a US soldier).
'caught' becomes 'c' (cricket abbreviation).
'lo'+'gi'+'c'='LOGIC'
(Other definitions for logic that I've seen before include "Branch of philosophy that analyses inference" , "using premises" , "Reasoned thinking" , "Science of correct reasoning" , "Sound sense" .)