Vapid chatter to flag up (3)
I believe the answer is:
gas
'vapid chatter' is the definition.
(gas can mean casual talk)
'flag up' is the wordplay.
'flag' becomes 'sag' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'sag' in reverse letter order is 'GAS'.
'to' is the link.
(Other definitions for gas that I've seen before include "eg Helium" , "Americans' petrol" , "Fumes, vapour" , "Talk profusely" , "16 (US)" .)