Most boring Washington DC airport, plus its terminal? (7)
I believe the answer is:
dullest
'most boring' is the definition.
('dull' can be similar in meaning to 'boring')
'washington dc airport plus its terminal?' is the wordplay.
I cannot quite understand how this works, but
'airport' could be 'dulles' (Dulles International Airport in Washington DC) and 'dulles' is found within the answer.
A single letter 't' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for dullest that I've seen before include "Most tedious" , "that's not at all interesting" , "Least shiny" , "Least interesting" , "Most unintelligent" .)