Passionate type of romantic relationship is doomed, initially (6)
I believe the answer is:
torrid
'passionate' is the definition.
(similar in meaning)
'type of romantic relationship is doomed initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'type of romantic relationship is doomed' is 'TORRID'.
(Other definitions for torrid that I've seen before include "Blistering - steamy" , "Highly emotional" , "Highly charged emotionally" , "Of climate, very hot and dry" , "Hot and emotional" .)