Still tiptop happening (5)
I believe the answer is:
event
'happening' is the definition.
(I've seen this in other clues)
'still tiptop' is the wordplay.
'still' becomes 'even' (synonyms).
'tiptop' becomes 't' (top of the word 'tip').
'even'+'t'='EVENT'
(Other definitions for event that I've seen before include "heat, perhaps" , "A noteworthy happening or occurrence" , "contest in programme" , "Meeting" , "Notable occasion" .)