Attempt to stop tree going west — or shrub (6)
I believe the answer is:
'shrub' is the definition.
(myrtle is a kind of shrub)
'attempt to stop tree going west' is the wordplay.
'attempt to' becomes 'try' (I've seen this in other clues).
'stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'tree' becomes 'elm' (elm tree is a kind of tree).
'going west' is a reversal indicator (letters heading to the west or left).
'try' going within 'elm' is 'eltrym'.
'eltrym' backwards is 'MYRTLE'.
'or' is the link.
(Other definitions for myrtle that I've seen before include "Girl" , "Evergreen shrub - try elm (anag)" , "Evergreen shrub with blue-black berries" , "Shrub with white flowers and aromatic berries" , "Termly (anag.)" .)