Ring exercises -- a source of laughter (4)
I believe the answer is:
peal
'ring' is the definition.
('peal' can be a synonym of 'ring')
'exercises a source of laughter' is the wordplay.
'exercises' becomes 'pe'.
'source of' indicates taking the first letters.
The initial letter of 'laughter' is 'l'.
'pe'+'a'+'l'='PEAL'
(Other definitions for peal that I've seen before include "(Bells) ring" , "Ring (of bell), clap (of thunder)" , "Leap about to sound of bells" , "Pale over sound of bells" , "Loud ringing of bells" .)