Ventured to admit time was flying (6)
I believe the answer is:
darted
'was flying' is the definition.
(to dart can mean to rush or fly around)
'ventured to admit time' is the wordplay.
'ventured' becomes 'dared' (I've seen this before).
'to admit' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'dared' enclosing 't' is 'DARTED'.
(Other definitions for darted that I've seen before include "Dashed, flitted" , "Moved rapidly and lightly" , "Moved suddenly like game missile" , "Was fleet" , "Moved quickly and suddenly" .)