Ambassador having pretentious tendencies is jovial (6)
I believe the answer is:
hearty
'is jovial' is the definition.
(I've seen this in another clue)
'ambassador having pretentious' is the wordplay.
'ambassador' becomes 'HE' (His or Her Excellency).
'having' says to put letters next to each other.
'pretentious' becomes 'arty' (similar in meaning).
'he'+'arty'='HEARTY'
'tendencies' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for hearty that I've seen before include "Wholesome and substantial, meal say" , "Genial" , "Vigorous and in good spirits, being earthy" , "Substantial and nourishing" , "Vigorously warm and friendly" .)