Thick skin envelops cubic body mass (8)
I believe the answer is:
calculus
'body mass' is the definition.
The definition and answer can be both natural objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'thick skin envelops cubic' is the wordplay.
'thick skin' becomes 'callus'.
'envelops' means one lot of letters goes inside another (I've seen this in other clues).
'cubic' becomes 'cu' (I can't explain this - if you can you should believe this answer much more).
'callus' going around 'cu' is 'CALCULUS'.
Can you help me to learn more?
(Other definitions for calculus that I've seen before include "Branch of maths involving rates of variation" , "Maths with derivatives" , "System of mathematical computation (often differential)" , "Branch of maths dealing with differentiation and integration" , "Maths dealing with rates of variation" .)