The foreign lieutenant in romance revealing a betrayal
I believe the answer is:
telltale
'a betrayal' is the definition.
(thesaurus)
'the foreign lieutenant in romance' is the wordplay.
'the foreign' becomes 'el' ('the' in Spanish).
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'in' indicates putting letters inside.
'romance' becomes 'tale' (I've seen this in another clue).
'el'+'lt'='ellt'
'ellt' placed into 'tale' is 'TELLTALE'.
'revealing' is the link.
(Other definitions for telltale that I've seen before include "Person revealing secrets" , "Outward indication of something concealed" , "Person who gossips or blabs secrets" , "Blabbermouth, snitch" , "Revealing indicator or sneak" .)