Regularly seen up in Dunnet Forest (5)
I believe the answer is:
often
'regularly' is the definition.
(often means regularly or frequently)
'up in dunnet forest' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' says the answer is hidden in the clue.
'netfo' is hidden in the letters of 'dunnet forest'.
'netfo' reversed gives 'OFTEN'.
'seen' is the link.
(Other definitions for often that I've seen before include "More than occasionally" , "Much of the time" , "In many cases" , "Regularly" , "Repeatedly, many times" .)