It reveals excitement as we pick up ambassador (4)
I believe the answer is:
whee
'it reveals excitement' is the definition.
(expression of excitement)
'we pick up ambassador' is the wordplay.
'pick up' means one lot of letters goes inside another.
'ambassador' becomes 'HE' (His or Her Excellency).
'we' enclosing 'he' is 'WHEE'.
'as' acts as a link.
(Other definitions for whee that I've seen before include "Expression of delight" , "What fun" , "I'm going downhill, fast!" , "Cry from someone going downhill fast?" , "Exuberant cry" .)