Train reaching Londonderry empty? Absolutely (9)
I believe the answer is:
expressly
'absolutely' is the definition.
(thesaurus)
'train reaching londonderry empty?' is the wordplay.
'train' becomes 'express' (express train is a kind of train**).
'reaching' is a charade indicator (letters next to each other).
'empty?' suggests removing the centre (I've seen 'empty' mean this).
'londonderry' with its middle removed is 'ly'.
'express'+'ly'='EXPRESSLY'
(Other definitions for expressly that I've seen before include "specifically" , "Explicitly" , "definitely" , "With specific intentions" , "unmistakably" .)