Retreat back to part of Venice (6)
I believe the answer is:
rialto
'part of venice' is the definition.
'retreat back to' is the wordplay.
'retreat' becomes 'lair' (I've seen this before).
'back' says the letters should be written in reverse.
'lair' written backwards gives 'rial'.
'rial'+'to'='RIALTO'
(Other definitions for rialto that I've seen before include "Venetian district" , "Venice area with famous bridge" , "Dublin district, or Shakespeare's 'What news on the . . . . . .?'" , "District in Venice and in south Dublin" , "Famous Venetian bridge" .)