Delightful excuse to celebrate (8)
I believe the answer is:
pleasing
'delightful' is the definition.
('pleasing' can be similar in meaning to 'delightful')
'excuse to celebrate' is the wordplay.
'excuse' becomes 'plea' (I've seen this before).
'to celebrate' becomes 'sing' (I've seen this before).
'plea'+'sing'='PLEASING'
(Other definitions for pleasing that I've seen before include "Pleasurable; satisfying" , "Delightful" , "making one happy" , "Satisfying or appealing" , "agreeable - with 13d" .)