Thailand omitted from maps, sadly (4)
I believe the answer is:
alas
'sadly' is the definition.
('alas' is used to express regret)
'thailand omitted from maps' is the wordplay.
'thailand' becomes 'T' (abbreviation on car registration plates).
'omitted from' indicates named letters should be taken away.
'maps' becomes 'atlas' (printed collection of maps).
'atlas' with 't' taken out is 'ALAS'.
(Other definitions for alas that I've seen before include "Antipodean island state" , "Too bad, so sad" , "'- Poor Yorick' (Hamlet)" , "woebegone expression" , "Oh dear!" .)