German cultivates rose bush (5)
I believe the answer is:
gorse
'bush' is the definition.
(gorse is a kind of bush)
'german cultivates rose' is the wordplay.
'german' becomes 'g' (abbreviation).
'cultivates' indicates anagramming the letters (I've seen 'cultivating' mean this).
'rose' anagrammed gives 'orse'.
'g'+'orse'='GORSE'
(Other definitions for gorse that I've seen before include "Welsh assembly" , "Moorland shrub" , "Yellow-flowering shrub" , "I'm always blooming" , "Prickly shrub, furze, whin" .)