Suffer endless public transport nightmare (7)
I believe the answer is:
incubus
'nightmare' is the definition.
(I know that nightmare can be written as incubus)
'suffer endless public transport' is the wordplay.
'suffer' becomes 'incur' (to incur is to suffer or experience).
'endless' means to remove the last letter.
'public transport' becomes 'bus' (bus is a kind of public transport).
'incur' with its last letter taken away is 'incu'.
'incu'+'bus'='INCUBUS'
(Other definitions for incubus that I've seen before include "Burdensome person or evil spirit visiting sleeper" , "Betraying Apostle" , "Evil spirit visiting a sleeping person" , "Male demon - nightmare - obsession" , "Demon bringing nightmares" .)