Ambassador on time finds article (3)
I believe the answer is:
the
'article' is the definition.
(the definite article in English)
'ambassador on time' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'on' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'he' put after 't' is 'THE'.
'finds' acts as a link.
(Other definitions for the that I've seen before include "See 10 Across" , "and 24 Down: Similar things" , "15, 16 One given taste of own medicine" , "That particular one" , "Finis" .)