It's in outer Ealing, that's true (4)
I believe the answer is:
real
'true' is the definition.
(similar in meaning)
'it's in outer ealing that's' is the wordplay.
'it's in' says the answer is hidden in the clue.
'REAL' is hidden within 'outer ealing that'.
(Other definitions for real that I've seen before include "The old coin actually exists" , "In America, this estate comprises land and buildings" , "True" , "Genuine - old Spanish coin" , "Actual, not imaginary" .)