Slough is in Berks, he declared (4)
I believe the answer is:
shed
'slough' is the definition.
('shed' can be a synonym of 'slough')
'in berks he declared' is the wordplay.
'in' says the answer is hidden in the clue.
'SHED' can be found hidden inside 'berks he declared'.
'is' is the link.
(Other definitions for shed that I've seen before include "Slough - outbuilding" , "place for bikes" , "Abandoned" , "shelter" , "Drop - cast off" .)