Good form of transport that was sought by knights (5)
I believe the answer is:
'transport that was sought by knights' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'good form' is the wordplay.
'good' becomes 'g' (abbreviation).
'form' becomes 'rail' (I have seen 'Form of transport ' mean 'rail' so perhaps 'form' could also mean 'rail').
'of' is the link.
Can you help me to learn more?
(Other definitions for grail that I've seen before include "Cherished goal" , "The Holy ..., the legendary chalice used at the Last Supper" , "Chalice used by Jesus at the Last Supper" , "Object of King Arthur's quest" , "Fabled holy vessel" .)