Good public transport system is the ultimate aim (5)
I believe the answer is:
grail
'the ultimate aim' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'good public transport system' is the wordplay.
I cannot quite understand how this works, but
'good' could be 'g' (abbreviation) and 'g' is located in the answer.
'transport system' could be 'rail' (I've seen this in another clue) and 'rail' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for grail that I've seen before include "Round Table Knights sought the Holy ....." , "Arthurian knights' quest" , "sought by Parsifal" , "Legendary holy item" , "Cherished goal" .)