Feel acute embarrassment having legal document served on ambassador (6)
I believe the answer is:
writhe
'feel acute embarrassment' is the definition.
(I've seen this before)
'legal document served on ambassador' is the wordplay.
'legal document' becomes 'writ' (writ is a kind of legal document).
'served on' means one lot of letters go next to another.
'ambassador' becomes 'he' (His or Her Excellency).
'writ'+'he'='WRITHE'
'having' acts as a link.
(Other definitions for writhe that I've seen before include "Move sinuously" , "feel acute embarrassment" , "It's painful moving" , "6, maybe in agony" , "Wriggle" .)