Only about half of it is over a yard (5)
I believe the answer is:
metre
'over a yard' is the definition.
I can't tell whether this defines the answer.
'only about half of it' is the wordplay.
'only' becomes 'mere' (I've seen this in another clue).
'about' is an insertion indicator.
'half of' suggests halving the letters.
'it' cut in half is 't'.
'mere' placed around 't' is 'METRE'.
'is' is the link.
Can you help me to learn more?
(Other definitions for metre that I've seen before include "1.094 yards" , "Approximately 39 inches" , "Yard is smaller than this" , "measure of length" , "Poetic rhythm" .)