Penetration, in a sense (7)
I believe the answer is:
insight
'penetration' is the definition.
(I've seen this before)
'in a sense' is the wordplay.
'a sense' becomes 'sight' (sight is a kind of sense).
'in'+'sight'='INSIGHT'
(Other definitions for insight that I've seen before include "Special knowledge" , "Practical knowledge" , "Grasp" , "comprehension" , "vision" .)