Put down flags, turning to speak, and vanish (9)
I believe the answer is:
evaporate
'vanish' is the definition.
(I've seen this before)
'put down flags turning to speak' is the wordplay.
'put down flags' becomes 'pave' (to pave could mean to put down flagstones).
'turning' says the letters should be written in reverse.
'to' says to put letters next to each other.
'speak' becomes 'orate' (orating is a kind of speaking).
'pave' written backwards gives 'evap'.
'evap'+'orate'='EVAPORATE'
'and' acts as a link.
(Other definitions for evaporate that I've seen before include "Melt away" , "let off steam" , "Turn into vapour, lose moisture" , "Fade away gradually" , "Go from liquid to vapour" .)