Many a beast has bulk (4)
I believe the answer is:
mass
'bulk' is the definition.
('mass' can be a synonym of 'bulk')
'many a beast' is the wordplay.
'many' becomes 'm' (**).
'a beast' becomes 'ass' (I've seen this before).
'm'+'ass'='MASS'
'has' acts as a link.
(Other definitions for mass that I've seen before include "Heap; religious service" , "Throng" , "Church service - crowd" , "state, in short" , "Church rite; large group" .)