Recognised at once in 'Heart of Darkness' (5)
I believe the answer is:
known
'recognised' is the definition.
(thesaurus)
'at once in heart of darkness' is the wordplay.
'at once' becomes 'now' (I've seen this before).
'in' indicates putting letters inside.
'heart of' indicates the central letters.
The middle of 'darkness' is 'kn'.
'now' going within 'kn' is 'KNOWN'.
(Other definitions for known that I've seen before include "Famous" , "Widely recognised" , "Acknowledged" , "Apprehended mentally" , "Noted" .)