Measure just about half of it (5)
I believe the answer is:
metre
'measure' is the definition.
(metre is a kind of measure)
'just about half of it' is the wordplay.
'just' becomes 'mere' (I've seen this in other clues**).
'about' means one lot of letters goes inside another.
'half of' suggests halving the letters.
'it' halved is 't'.
'mere' enclosing 't' is 'METRE'.
(Other definitions for metre that I've seen before include "Rhythmic arrangement of syllables in verse" , "100cm" , "1.094 yards" , "Distance" , "Approx 39 inches" .)