Expensive year king held to be tiresome (6)
I believe the answer is:
dreary
'be tiresome' is the definition.
(synonyms)
'expensive year king held' is the wordplay.
'expensive' becomes 'dear' (similar in meaning).
'year' becomes 'y'.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'held' means one lot of letters goes inside another.
'dear'+'y'='deary'
'deary' enclosing 'r' is 'DREARY'.
'to' acts as a link.
(Other definitions for dreary that I've seen before include "Dismal, depressing" , "Dismal, funeral" , "Grey and dull" , "Gloomy - dull" , "Dark" .)