Work hard, gathering in fine wrapping material (7)
I believe the answer is:
tinfoil
'wrapping material' is the definition.
(material which food is wrapped in)
'work hard gathering in fine' is the wordplay.
'work hard' becomes 'toil' (to toil is to do strenuous work).
'gathering' indicates putting letters inside.
'fine' becomes 'f' (abbreviation).
'in'+'f'='inf'
'toil' going around 'inf' is 'TINFOIL'.
(Other definitions for tinfoil that I've seen before include "Food wrapper" , "Thin metallic sheet" , "Thin metal sheet for wrapping food etc" , "Thin metallic sheets for protecting food" , "Fine metal" .)