Secure space on way up (4)
I believe the answer is:
moor
'secure' is the definition.
(mooring is a kind of securing)
'space on way up' is the wordplay.
'space' becomes 'room' (enough space or room to do something).
'on way up' says the letters should be written in reverse.
'room' written backwards gives 'MOOR'.
(Other definitions for moor that I've seen before include "Open upland; tie up boat" , "Expanse of uncultivated upland" , "African - wasteland" , "Anchor" , "Room for Moroccan Muslim" .)