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' is an insertion indicator (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 into 'elm' is 'eltrym'.
'eltrym' back-to-front 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" .)