Be outside; I say I see why first class left a form of casual transport (7)
I believe the answer is:
bicycle
'transport' is the definition.
(I know that bicycle is a type of vehicle)
'be outside i say i see why first class left' is the wordplay.
'outside' means one lot of letters goes inside another (some letters go outside others).
'say i see' becomes 'c' (I can't justify this - if you can you should give a lot more credence to this answer).
'why' becomes 'y'.
'first' says to take the initial letters.
The initial letters of 'class left' is 'cl'.
'i'+'c'+'y'+'cl'='icycl'
'be' placed around 'icycl' is 'BICYCLE'.
'a form of casual' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for bicycle that I've seen before include "Personal transport" , "Lowest values" , "One's pushed to make progress" , "Two-wheeler" , "Pedalled vehicle" .)