Lecture head of state attending summit (5)
I believe the answer is:
speak
'lecture' is the definition.
(I've seen this in another clue)
'head of state attending summit' is the wordplay.
'head of' suggests taking the first letters.
'attending' says to put letters next to each other (to attend can mean to follow).
'summit' becomes 'peak' (mountain peak).
The initial letter of 'state' is 's'.
's'+'peak'='SPEAK'
(Other definitions for speak that I've seen before include "Utter words" , "Talk" , "Converse" , "give a lecture" , "express" .)