A relative getting up with cereal, ultimately for fibre (5)
I believe the answer is:
sisal
'for fibre' is the definition.
(sisal is a kind of fibre)
'a relative getting up with cereal ultimately' is the wordplay.
'relative' becomes 'sis' (short for sister).
'getting up' shows that the letters should be reversed in order.
'with' means one lot of letters go next to another.
'ultimately' suggests the final letters.
The last letter of 'cereal' is 'l'.
'a'+'sis'='asis'
'asis' in reverse letter order is 'sisa'.
'sisa'+'l'='SISAL'
(Other definitions for sisal that I've seen before include "Plant fibre used for ropes" , "Strong rope fibre" , "Natural fibre" , "Agave fibre found in bad sails" , "Fibre-yielding plant" .)