Retreat by rail, possibly? (4)
I believe the answer is:
lair
'retreat' is the definition.
(both can mean a private hideout)
'rail possibly?' is the wordplay.
'possibly?' is an anagram indicator (I've seen 'possibly' mean this).
'rail' anagrammed gives 'LAIR'.
'by' acts as a link.
(Other definitions for lair that I've seen before include "Rail around animal's den" , "enclosure" , "Supposed large ape, the Bigfoot" , "Waylay" , "Rail about wild animal's home" .)