Throne issue for PM and POTUS? (7)
I believe the answer is:
johnson
'pm and potus?' is the definition.
(surname of a British PM and two US presidents)
'throne issue' is the wordplay.
'throne' becomes 'john' (both can informally mean a toilet).
'issue' becomes 'son' (issue can mean someone's offspring).
'john'+'son'='JOHNSON'
'for' is the link.
(Other definitions for johnson that I've seen before include "Two US presidents; a literary Doctor" , "Lexicographic doctor" , "American president" , "Samuel -, lexicographer" , "Lexicographical doctor" .)