Pithy bard launched pretentious annual celebration (8,5)
I believe the answer is:
birthday party
'annual celebration' is the definition.
'pithy bard launched pretentious' is the wordplay.
'launched' indicates an anagram.
'pretentious' becomes 'arty' (similar in meaning).
'pithy'+'bard'='pithybard'
'pithybard' is an anagram of 'birthdayp'.
'birthdayp'+'arty'='BIRTHDAY PARTY'
(Other definitions for birthday party that I've seen before include "Annual knees-up?" , "Occasion for cake with candles" , "Celebration - Pinter play" .)