Popular copper to transport unwelcome nocturnal visitor (7)
I believe the answer is:
incubus
'unwelcome nocturnal visitor' is the definition.
(mythical demon visiting sleeping women)
'popular copper to transport' is the wordplay.
'popular' becomes 'in'.
'copper' becomes 'cu' (chemical symbol).
'to transport' becomes 'bus' (busing is a kind of transporting).
'in'+'cu'+'bus'='INCUBUS'
(Other definitions for incubus that I've seen before include "Oppressive nightmare" , "Demon bringing nightmares" , "Male demon - nightmare - obsession" , "Nightmare - cub in us (anag)" , "Burdensome person or evil spirit visiting sleeper" .)