Yellow flower improving serene rocks (7,8)
I believe the answer is:
evening primrose
'yellow flower' is the definition.
(I've seen this in another clue)
'improving serene rocks' is the wordplay.
'rocks' indicates an anagram.
'improving'+'serene'='improvingserene'
'improvingserene' is an anagram of 'EVENING-PRIMROSE'.
(Other definitions for evening primrose that I've seen before include "flowering plant" , "Bloomer" , "plant that flowers at night" , "flower" , "Herbal remedy" .)