Retreat regularly visited for period (3)
I believe the answer is:
era
'period' is the definition.
(I know that period can be written as era)
'retreat regularly visited' is the wordplay.
'regularly visited' means one should take alternating letters (letters are taken at regular intervals).
The alternate letters of 'retreat' are 'ERA'.
'for' is the link.
(Other definitions for era that I've seen before include "quite some time" , "Cycle" , "Are involved in time span" , "Long period of history" , "Important historical period" .)