Flower festival not starting (5)
I believe the answer is:
aster
'flower' is the definition.
(aster is a kind of flower)
'festival not starting' is the wordplay.
'festival' becomes 'easter' (I've seen this before).
'not starting' means to remove the first letter.
'easter' with its initial letter taken away is 'ASTER'.
(Other definitions for aster that I've seen before include "eg Michaelmas daisy" , "Plant like a Michaelmas daisy" , "It grows" , "Daisy-like garden flower" , "Plant of Michaelmas daisy family" .)