Common brown climbing flower (7)
I believe the answer is:
natural
'common' is the definition.
(thesaurus)
'brown climbing flower' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'flower' becomes 'ural' (I've seen this before).
'tan' reversed gives 'nat'.
'nat'+'ural'='NATURAL'
(Other definitions for natural that I've seen before include "Relaxed, unaffected" , "Expected - appropriate - artless" , "Additive-free" , "Normal - innate - instinctive" , "Without affectation" .)