Tendency to cover same ground in cellar (8)
I believe the answer is:
basement
'cellar' is the definition.
(synonyms)
'tendency to cover same ground' is the wordplay.
'tendency' becomes 'bent' (bent is a kind of tendency).
'to cover' is an insertion indicator.
'ground' indicates anagramming the letters ('grind' the letters into a new form).
'same' with letters rearranged gives 'asem'.
'bent' placed around 'asem' is 'BASEMENT'.
'in' is the link.
(Other definitions for basement that I've seen before include "Man beset in bottom of house" , "Bottom floor" , "Underground room" , "Lower ground floor" , "Room at the bottom" .)