Grand and endlessly thrilling setter (7)
I believe the answer is:
gelatin
'setter' is the definition.
(substance which sets or hardens)
'grand and endlessly thrilling' is the wordplay.
'grand' becomes 'g' (G is short for grand as in a thousand).
'and' says to put letters next to each other.
'endlessly' means to remove the last letter.
'thrilling' becomes 'elating' (to elate is to thrill or delight).
'elating' with its final letter taken off is 'elatin'.
'g'+'elatin'='GELATIN'
(Other definitions for gelatin that I've seen before include "Colourless protein extracted from animal bones" , "Food/glue jelly" , "Setter" , "Glue made from animal extracts" , "Jelling agent" .)