Good joint of meat containing a lot of roughage and protein (8)
I believe the answer is:
globulin
'protein' is the definition.
(globulin is a kind of protein)
'good joint of meat containing a lot of roughage' is the wordplay.
'good' becomes 'g' (abbreviation).
'joint of meat' becomes 'loin' (I've seen this before).
'containing' means one lot of letters goes inside another.
'a lot of roughage' becomes 'bul' (I can't justify this - if you can you should give a lot more credence to this answer).
'loin' placed around 'bul' is 'lobulin'.
'g'+'lobulin'='GLOBULIN'
'and' acts as a link.
Can you help me to learn more?
(Another definition for globulin that I've seen is " Protein found in blood".)