Donald Trump, the first American king (4)
I believe the answer is:
tusk
'donald' is the definition.
(Donald Tusk)
'trump the first american king' is the wordplay.
'the first' says to take the initial letters.
'american' becomes 'us'.
'king' becomes 'k'.
The first letter of 'trump' is 't'.
't'+'us'+'k'='TUSK'
(Other definitions for tusk that I've seen before include "'Long pointed tooth, of an elephant say (4)'" , "Large tooth" , "Jumbo tooth" , "Walrus's long tooth" , "Elephant's dentine" .)