Fast-growing trees, ideally spread across end of garden facing one (9)
I believe the answer is:
leylandii
'fast-growing trees' is the definition.
(type of tree)
'ideally spread across end of garden facing one' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found within the answer.
This explanation may well be incorrect...
(Other definitions for leylandii that I've seen before include "Fast-growing cypress" , "Variety of cypress" , "Tree -- fast-growing conifer" , "(Very) fast-growing conifer(s)" , "Fast-growing plant" .)