Frank's seeing that Republican leader in extremis (7)
I believe the answer is:
sincere
'frank's' is the definition.
(frank can mean sincere or honest)
'seeing that republican leader in extremis' is the wordplay.
'seeing that' becomes 'since' (both can mean 'when considering...').
'republican' becomes 'R' (political abbreviation).
'leader in' suggests taking the first letters.
The first letter of 'extremis' is 'e'.
'since'+'r'+'e'='SINCERE'
(Other definitions for sincere that I've seen before include "Pure" , "Showing genuine feelings" , "straightforward" , "Unfeigned, genuine" , "Without deceit, genuine" .)