Float starts to wobble about -- festival thrills! (4)
I believe the answer is:
waft
'float' is the definition.
(I've seen this before)
'starts to wobble about festival thrills' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'wobble about festival thrills' is 'WAFT'.
(Other definitions for waft that I've seen before include "Be carried on the breeze" , "Cause to float through the air" , "Float on breeze" , "Sail" , "Gentle breeze" .)