Dismal king in love (5)
I believe the answer is:
drear
'dismal' is the definition.
(thesaurus)
'king in love' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'in' indicates putting letters inside.
'love' becomes 'dear' ('dear' can be a synonym of 'love'**).
'r' going into 'dear' is 'DREAR'.
(Other definitions for drear that I've seen before include "Grim" , "Morose" , "Literarily gloomy" , "Gloomy and cheerless" , "grave" .)