Work hard on back of durable fabric (5)
I believe the answer is:
toile
'fabric' is the definition.
(I've seen this in another clue)
'work hard on back of durable' is the wordplay.
'work hard' becomes 'toil' (to toil is to do strenuous work).
'on' says to put letters next to each other.
'back of' says to take the final letters.
The last letter of 'durable' is 'e'.
'toil'+'e'='TOILE'
(Other definitions for toile that I've seen before include "Fabric" , "dress material" , "Worked hard" , "6 [LINEN]" , "cheap prototype" .)