Shellfish 100 pounds a thousand (4)
I believe the answer is:
clam
'shellfish' is the definition.
(I've seen this before)
'100 pounds a thousand' is the wordplay.
'm00' becomes 'c' (roman numeral).
'pounds' becomes 'l' (L can represent the monetary pound).
'thousand' becomes 'm' ('m' can be a synonym of 'thousand'**).
'c'+'l'+'a'+'m'='CLAM'
(Other definitions for clam that I've seen before include "Shellfish - secretive person" , "Mollusc, one proverbially reticent" , "Proverbially reticent marine mollusc" , "ringing noise produced by > very reticent type" , "Aquatic creature" .)