A German man’s climbing plants (5)
I believe the answer is:
flora
'plants' is the definition.
(I've seen this before)
'a german man's climbing' is the wordplay.
'german man' becomes 'Rolf' (common German name).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'a'+'rolf'='arolf'
'arolf' reversed gives 'FLORA'.
(Other definitions for flora that I've seen before include "list of plants" , "Botanical specimens" , "collection of plants" , "A region's plants" , "vegetation" .)