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