Tissue a certain Frenchman chucked into grave (4,6)
I believe the answer is:
bone marrow
'tissue' is the definition.
(bone marrow is a kind of tissue)
'a certain frenchman chucked into grave' is the wordplay.
'a certain' becomes 'one' (I've seen this before).
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'chucked into' means one lot of letters goes inside another.
'grave' becomes 'barrow' (I've seen this before).
'one'+'m'='onem'
'onem' put inside 'barrow' is 'BONE MARROW'.
(Other definitions for bone marrow that I've seen before include "Where blood cells are manufactured" , "transplant tissue" , "Producer of blood cells" , "Vital part of skeleton" , "Fatty network of connective tissue" .)