Olive tree's beginning to develop flower (6)
I believe the answer is:
violet
'flower' is the definition.
(thesaurus)
'olive tree's beginning to develop' is the wordplay.
'beginning to' suggests taking the first letters.
'develop' indicates an anagram.
The first letter of 'tree' is 't'.
'olive'+'t'='olivet'
'olivet' anagrammed gives 'VIOLET'.
(Other definitions for violet that I've seen before include "Bloomer" , "Seldom" , "Mauve" , "Bluish purple" , "See 9" .)