Knowing in retrospect something of Eric, it's on guitar (7)
I believe the answer is:
gnostic
'knowing' is the definition.
(thesaurus)
'in retrospect something of eric it's on guitar' is the wordplay.
'in retrospect' says the letters should be written backwards.
'something of' indicates a hidden word (I've seen 'something' mean this (I've seen this in other clues)).
'citsong' can be found hidden inside 'eric its on guitar'.
'citsong' written backwards gives 'GNOSTIC'.
(Other definitions for gnostic that I've seen before include "Relating to mystical knowledge" , "Having special mystical knowledge, costing a lot" , "Occult" .)