Fibre one’s covered with tar mostly (5)
I believe the answer is:
sisal
'fibre' is the definition.
(sisal is a kind of fibre)
'one's covered with tar mostly' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'covered with' is an insertion indicator.
'tar' becomes 'salt' (synonyms).
'mostly' means to remove the last letter.
'salt' with its last letter taken off is 'sal'.
'is' placed into 'sal' is 'SISAL'.
(Other definitions for sisal that I've seen before include "Fibre of Mexican agave" , "Hemp fibre" , "Agave yielding fibre" , "Fibre from agave plant" , "Rope-fibre made from leaves of tropical plant" .)