Wisdom of a copper, soldiers (6)
I believe the answer is:
acumen
'wisdom' is the definition.
(good or wise judgement)
'a copper soldiers' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'soldiers' becomes 'men' (I've seen this before).
'a'+'cu'+'men'='ACUMEN'
'of' acts as a link.
(Other definitions for acumen that I've seen before include "Keen intelligence" , "Being sharp" , "Sharpness, penetration, quickness of perception" , "show perception" , "ingenuity" .)