Insightfulness (6)
I believe the answer is:
acumen
'insightfulness' is the definition.
(synonyms)
This is all the clue.
(Other definitions for acumen that I've seen before include "Keen insight, shrewdness" , "ability to make good judgements" , "Keen intelligence" , "Ability to take well judged decisions" , "'Shrewdness, in business perhaps (6)'" .)