Myth was first to include knowledge (6)
I believe the answer is:
legend
'myth' is the definition.
(I've seen this before)
'was first to include knowledge' is the wordplay.
'was first' becomes 'led' (to lead is to go first).
'to include' means one lot of letters goes inside another.
'knowledge' becomes 'gen' (I've seen this before).
'led' going around 'gen' is 'LEGEND'.
(Other definitions for legend that I've seen before include "Very famous member's death" , "extremely famous person?" , "Stanley Matthews, perhaps" , "famous player?" , "Old tale" .)