Romantic thought is introduced to lieutenant (8)
I believe the answer is:
idealist
'romantic' is the definition.
(thesaurus)
'thought is introduced to lieutenant' is the wordplay.
'thought' becomes 'idea' ('idea' can be a synonym of 'thought'**).
'introduced to' means one lot of letters goes inside another.
'lieutenant' becomes 'lt'.
'is' put within 'lt' is 'list'.
'idea'+'list'='IDEALIST'
(Other definitions for idealist that I've seen before include "Seeker after perfection" , "One sees things as they should be rather than how they are" , "Person with noble aspirations, perhaps unrealistic" , "One who pursues the perfect" , "Person with lofty and unselfish aspirations" .)