L. S. Lowry occasionally chanced sweet course (6)
I believe the answer is:
sorbet
'sweet course' is the definition.
'l s lowry occasionally chanced' is the wordplay.
'occasionally' means one should take alternating letters.
'chanced' becomes 'bet'.
'l'+'s'+'lowry'='lslowry'
The alternate letters of 'lslowry' are 'sor'.
'sor'+'bet'='SORBET'
(Other definitions for sorbet that I've seen before include "Mushy ice drink to cleanse the palate" , "Strobe (anag.)" , "Sweet" , "Water ice to cleanse the palate" , "Fruit water-ice" .)