Jumper mostly sported at entrance to tyre plant (8)

'plant' is the definition.
(fleawort is a kind of plant)

'jumper mostly sported at entrance to tyre' is the wordplay.
'jumper' becomes 'flea' (something that jumps).
'mostly' means to remove the last letter.
'sported' becomes 'wore' (I've seen this before).
'at' means one lot of letters go next to another.
'entrance to' says to take the initial letters.
The first letter of 'tyre' is 't'.
'wore' with its last letter taken away is 'wor'.
'flea'+'wor'+'t'='FLEAWORT'