Rough stuff, one caught by filly that's rolled over (5)
I believe the answer is:
sisal
'rough stuff' is the definition.
(rough material)
'one caught by filly that's rolled over' is the wordplay.
'one' becomes 'i' (Roman numeral).
'caught by' is an insertion indicator.
'filly' becomes 'lass' (I've seen this in another clue).
'that's rolled over' shows that the letters should be reversed in order.
'i' placed within 'lass' is 'lasis'.
'lasis' in reverse letter order is 'SISAL'.
(Other definitions for sisal that I've seen before include "material for painter, say?" , "Type of fibre" , "Plant fibre used for making rope" , "Plant producing rope" , "Fibrous plant used for rope" .)