A cart led by good knight (6)
I believe the answer is:
gawain
'knight' is the definition.
(Sir Gawain is a knight in Arthurian legend)
'a cart led by good' is the wordplay.
'cart' becomes 'wain' (I've seen this before).
'led by' means one lot of letters go next to another (some letters go in front of others).
'good' becomes 'g' (abbreviation).
'a'+'wain'='awain'
'awain' put after 'g' is 'GAWAIN'.
(Other definitions for gawain that I've seen before include "K" , "Arthurian knight of the Round Table" , "Man at round table" , "One of King Arthur's knights" , "round a table for him and his comrades?" .)