Fibre displayed by boy supporting little sibling (5)
I believe the answer is:
sisal
'fibre' is the definition.
(I know that sisal is a type of plant fiber)
'boy supporting little sibling' is the wordplay.
'boy' becomes 'sal' ().
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'little' means to remove the last letter (the word is not quite complete).
'sibling' becomes 'sis' (short for sister).
'sis' with its final letter removed is 'si'.
'sal' put after 'si' is 'SISAL'.
'displayed by' is the link.
(Other definitions for sisal that I've seen before include "Plant fibre used to make string" , "material for painter, say?" , "Fibre from agave plant" , "Rope fibre from leaves of tropical plant" , "Hemp fibre" .)