Tissue transplant is hard work (5)
I believe the answer is:
graft
I believe this clue is a double definition.
'tissue transplant' is the first definition.
(as in a skin graft)
'is hard work' is the second definition.
(graft can mean working hard)
(Other definitions for graft that I've seen before include "Means of propagation - hard work?" , "Hard work - corrupt profit" , "Hard work; bribery" , "Transfer of tissue - hard work!" , "Hard work - illicit gain" .)