Imbecile master carries stack onto beast (6)
I believe the answer is:
lummox
'imbecile' is the definition.
'master carries stack onto beast' is the wordplay.
'master' becomes 'm'.
'carries' is a charade indicator (letters next to each other) (in a down clue, some letters go beneath others).
'stack' becomes 'lum' (both can mean a chimney).
'onto' means one lot of letters go next to another.
'beast' becomes 'ox'.
'm' put after 'lum' is 'lumm'.
'lumm'+'ox'='LUMMOX'
(Other definitions for lummox that I've seen before include "Lumbering oaf" , "Gauche fool" , "Ass" , "Clumsy oaf" .)