But we must supplant president's last love (7)
I believe the answer is:
however
'but' is the definition.
(thesaurus)
'we must supplant president's last love' is the wordplay.
I cannot quite see how this works, but
'we' is present in the answer.
'love' could be 'o' (In tennis, 'love' means 'zero') and 'o' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for however that I've seen before include "In whatever way" , "Nevertheless" , "at all events" , "Yet" , "Notwithstanding" .)