Over a yard, but just about a quarter of a foot! (5)
I believe the answer is:
metre
'over a yard' is the definition.
I can't tell whether this definition defines the answer.
'just about a quarter of a foot' is the wordplay.
'just' becomes 'mere' ('just' can mean 'mere' or 'only').
'about' is an insertion indicator.
'a quarter of a foot' becomes 't' (I can't explain this - if you can you should believe this answer much more).
'mere' going around 't' is 'METRE'.
'but' is the link.
Can you help me to learn more?
(Other definitions for metre that I've seen before include "Poetic rhythm" , "Distance" , "100cm" , "Approximately 39 inches" , "three and a bit of a fourth" .)