I'll say it's not accessible by car (4)
I believe the answer is:
isle
'not accessible by car' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'i'll say' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'ill' sounds like 'ISLE'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for isle that I've seen before include "eg Man, Ely" , "Man conceivably" , "one ringed in the main?" , "Land encircled by water" , "Wight or Man, say" .)