It's just outside central Acton, not far (5)
I believe the answer is:
metre
'not far' is the definition.
I can't tell whether this defines the answer.
'it's just outside central acton' is the wordplay.
'it's just' becomes 'mere' ('just' can mean 'mere' or 'only').
'outside' is an insertion indicator (some letters go outside others).
'central' says to take the centre.
The middle of 'acton' is 't'.
'mere' placed around 't' is 'METRE'.
Can you help me to learn more?
(Other definitions for metre that I've seen before include "Moisture remover" , "Poetic rhythm" , "Approximately 39 inches" , "measure of length" , "1.094 yards" .)