Aware of Heart of Darkness: outstanding (7)
I believe the answer is:
knowing
'aware of' is the definition.
(I've seen this before)
'heart of darkness outstanding' is the wordplay.
'heart of' means to look at the middle letters.
'outstanding' becomes 'owing' (an outstanding bill is still owing).
The middle letters of 'darkness' are 'kn'.
'kn'+'owing'='KNOWING'
(Other definitions for knowing that I've seen before include "Cunning" , "Understanding" , "Arch" , "Meaningful" , "Wise" .)