Initially, greasing roast with cooking fat is hard work (5)
I believe the answer is:
graft
'is hard work' is the definition.
(graft can mean working hard)
'initially greasing roast with cooking fat' is the wordplay.
'initially' says to take the initial letters.
'with cooking' indicates an anagram.
The initial letters of 'greasing roast' is 'gr'.
'fat' with letters rearranged gives 'aft'.
'gr'+'aft'='GRAFT'
(Other definitions for graft that I've seen before include "Hard work - swindle" , "Hard work - corrupt profit" , "Transplant - hard work" , "Means of propagation - hard work?" , "Hard work; corruption" .)