All there is beneath fertiliser staff dug in (6,6)
I believe the answer is:
compos mentis
'all there' is the definition.
(both can mean mentally sound)
'is beneath fertiliser staff dug in' is the wordplay.
'beneath' means one lot of letters go next to another (in a down clue).
'fertiliser' becomes 'compost' (type of fertiliser).
'staff' becomes 'men' (I've seen this in other clues).
'dug in' is an insertion indicator (some letters dig their way into the word).
'compost' going around 'men' is 'composment'.
'is' put after 'composment' is 'COMPOS MENTIS'.
(Other definitions for compos mentis that I've seen before include "Of sound mind (Latin)" , "Capable of reasoning" , "Sane" .)