Alpha Centauri's heading about space (4)
I believe the answer is:
acre
'space' is the definition.
(acre is a unit of area or space)
'alpha centauri's heading about' is the wordplay.
'heading' says to take the initial letters (head could mean the top letter).
'about' becomes 're' ('re' means regarding or about).
The initial letters of 'alpha centauri' is 'ac'.
'ac'+'re'='ACRE'
(Other definitions for acre that I've seen before include "point four nought five of a hectare" , "Unit of land measurement" , "Not a lot of land" , "Unit of area - Israeli port" , "Race (anag.)" .)