Having secret knowledge, lied perhaps about credit being short (7)
I believe the answer is:
gnostic
'having secret knowledge' is the definition.
(gnostic means relating to mystical knowledge)
'lied perhaps about credit being short' is the wordplay.
'lied perhaps' becomes 'song' (lied is a kind of song).
'about' shows that the letters should be reversed in order.
'credit being short' becomes 'tic' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'song' in reverse letter order is 'gnos'.
'gnos'+'tic'='GNOSTIC'
(Other definitions for gnostic that I've seen before include "Relating to mystical knowledge" , "Having special mystical knowledge, costing a lot" , "Occult" .)