Good form of transport that was sought by knights (5)
I believe the answer is:
grail
'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').
'g'+'rail'='GRAIL'
'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" .)