Timmy discovered June won't start, not being impressionable (6)
I believe the answer is:
immune
'not being impressionable' is the definition.
I can't tell whether this definition defines the answer.
'timmy discovered june won't start' is the wordplay.
'discovered' says to take the centre.
'won't start' means to remove the first letter.
The central letters of 'timmy' are 'imm'.
'june' with its initial letter taken away is 'une'.
'imm'+'une'='IMMUNE'
Can you help me to learn more?
(Other definitions for immune that I've seen before include "Exempt, invulnerable" , "Safe from infection etc" , "Secure against, infection say" , "Resistant (to infection)" , "Not liable to, perhaps disease" .)