Observe heads of state expressing enthusiasm (3)
I believe the answer is:
see
'observe' is the definition.
(observe can mean to see something)
'heads of state expressing enthusiasm' is the wordplay.
'heads of' says to take the initial letters.
The initial letters of 'state expressing enthusiasm' is 'SEE'.
(Other definitions for see that I've seen before include "Salisbury, for one" , "Consider; diocese" , "Three points" , "Diocese of a bishop" , "District of a bishop - or Pope" .)