Plant gardener's first rambling rose (5)
I believe the answer is:
gorse
'plant' is the definition.
The answer is an example.
'gardener's first rambling rose' is the wordplay.
'gardener's first' becomes 'g' (1st letter of 'gardener').
'rambling' indicates an anagram.
'rose' is an anagram of 'orse'.
'g'+'orse'='GORSE'
(Other definitions for gorse that I've seen before include "Spiny evergreen shrub" , "Yellow-flowering shrub" , "Spiny yellow-flowered shrub" , "Ogres (anag.)" , "I'm always blooming" .)