Ambassador joining father, getting twitch of the liver (7)
I believe the answer is:
hepatic
'of the liver' is the definition.
(I've seen this in another clue)
'ambassador joining father getting twitch' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'joining' is a charade indicator (letters next to each other).
'father' becomes 'pa' (pa can mean dad or father).
'getting' says to put letters next to each other.
'twitch' becomes 'tic' (tic is a kind of twitch).
'he'+'pa'+'tic'='HEPATIC'
(Other definitions for hepatic that I've seen before include "Pertaining to, or affecting, the liver or liverwort" , "brownish-red" , "of an organ" , "Relating to the liver" , "Livery" .)