Philosopher and ambassador joining set (5)
I believe the answer is:
hegel
'philosopher' is the definition.
(German philosopher Georg Wilhelm Friedrich Hegel)
'ambassador joining set' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'joining' is a charade indicator (letters next to each other).
'set' becomes 'gel' (both can mean to harden).
'he'+'gel'='HEGEL'
'and' is the link.
(Other definitions for hegel that I've seen before include "German 'dialectic' philosopher" , "Georg Wilhelm ---, Ger. philosopher" , "German philosopher, d. 1831" .)