Entering allotment, drop rock (8)
I believe the answer is:
dolomite
'rock' is the definition.
(I know that dolomite is a type of rock)
'entering allotment drop' is the wordplay.
'entering' indicates putting letters inside.
'allotment' becomes 'dole' (I've seen this in another clue).
'drop' becomes 'omit' (synonyms).
'dole' enclosing 'omit' is 'DOLOMITE'.
(Other definitions for dolomite that I've seen before include "Limestone rock" , "Mineral after which a range of Italian Alps is named" , "stone" , "Form of rock" .)