Endeavour to cut tree over shrub (6)
I believe the answer is:
myrtle
'shrub' is the definition.
(myrtle is a kind of shrub)
'endeavour to cut tree over' is the wordplay.
'endeavour' becomes 'try' (synonyms).
'to cut' indicates putting letters inside (some letters cut their way into a word).
'tree' becomes 'elm' (elm tree is a kind of tree).
'over' says the letters should be written backwards.
'try' inserted within 'elm' is 'eltrym'.
'eltrym' in reverse letter order is 'MYRTLE'.
(Other definitions for myrtle that I've seen before include "Girl" , "Plant with aromatic blue-black berries" , "Shiny evergreen shrub" , "Fragrant evergreen shrub" , "Shrub; female name" .)