Measure space rock that's out of the ordinary (5)
I believe the answer is:
meter
'measure' is the definition.
(I know that measure can be written as meter)
'space rock that's out of the ordinary' is the wordplay.
'space rock' becomes 'meteor' (rock flying through space).
'that's out of' indicates named letters should be taken away.
'the ordinary' becomes 'O' (abbreviation e.g. O-Levels).
'meteor' with 'o' removed is 'METER'.
(Other definitions for meter that I've seen before include "Apparatus for measuring" , "Device that measures, say, electricity usage" , "Device for recording, say, usage (5)" , "Gauge" , "Device for measuring and recording" .)