Good morning Cruciverba ,I have been staring at it too ! I finished yesterday and hallelujah I think I’ve got it. Quite complicated but very clever . The answer reads as a word in the clue without its head which happens to be abbreviation for ambassador .
FOI 7d LOI 26a COD has to be 26a but liked 5d too
In fact quite a few contenders , brilliant prize crossword ,thank you Pasquale