There's nothing back in Fylingdales (3)
I believe the answer is:
nil
'there's nothing' is the definition.
(I've seen this before)
'back in fylingdales' is the wordplay.
'back' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'lin' is hidden within 'fylingdales'.
'lin' in reverse letter order is 'NIL'.
(Other definitions for nil that I've seen before include "Nothing - in Latin" , "Nothing, zero" , "Lacking value" , "You get nothing" , "There's nothing left" .)