Fibre left behind court (4)
I believe the answer is:
wool
'fibre' is the definition.
(wool is a kind of fibre)
'left behind court' is the wordplay.
'left' becomes 'l' (common abbreviation).
'behind' says to put letters next to each other.
'court' becomes 'woo' ('woo' can be a synonym of 'court'**).
'l' put after 'woo' is 'WOOL'.
(Other definitions for wool that I've seen before include "Fabric made from the hair of sheep" , "Ovine product" , "Yarn for knitting" , "What can be pulled over eyes to deceive" , "Yarn spun" .)