Old stone tool, one for garden holding most of work up (6)
I believe the answer is:
eolith
'old stone' is the definition.
(type of ancient stone formation)
'tool one for garden holding most of work up' is the wordplay.
'tool one for garden' becomes 'hoe' (tool for use in the garden).
'holding' is an insertion indicator.
'most of work' becomes 'til' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'up' is a reversal indicator (in down clue: letters go upwards).
'hoe' enclosing 'til' is 'htiloe'.
'htiloe' reversed gives 'EOLITH'.
(Other definitions for eolith that I've seen before include "Old stone implement" , "Very early stone implement to be found in the oil" , "stone tool" , "Elvis Presley song" , "Flint" .)