Ring friend about opening of exhibition (4)
I believe the answer is:
peal
'ring' is the definition.
(pealing is a kind of ringing)
'friend about opening of exhibition' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'about' indicates putting letters inside.
'opening of' indicates taking the first letters.
The first letter of 'exhibition' is 'e'.
'pal' placed around 'e' is 'PEAL'.
(Other definitions for peal that I've seen before include "Leap up to loud ring of a bell" , "Loud burst of thunder - or laughter" , "Set of church bells" , "Sound of ringing bells" , "Sound of bells or laughter" .)