Flat in Ruislip a dump (3)
I believe the answer is:
pad
'flat' is the definition.
(a person's home)
'in ruislip a dump' is the wordplay.
'in' says the answer is hidden in the clue.
'PAD' can be found hidden inside 'ruislip a dump'.
(Other definitions for pad that I've seen before include "writer's block" , "Colloquial apartment or soft bundle" , "Stuff the animal's foot" , "Absorbent material" , "Residence" .)