A poet inhaling bit of opium on a train? (6)
I believe the answer is:
aboard
'on a train?' is the definition.
(I've seen this before)
'a poet inhaling bit of opium' is the wordplay.
'poet' becomes 'bard' (bard is a kind of poet).
'inhaling' indicates putting letters inside.
'bit of' says to take the initial letters.
The initial letter of 'opium' is 'o'.
'bard' enclosing 'o' is 'board'.
'a'+'board'='ABOARD'
(Other definitions for aboard that I've seen before include "ready to take off" , "on boat, perhaps" , "On vessel or vehicle" , "on base" , "Joined for mutual benefit" .)