Happy soul flourished when holding single flower (9)
I believe the answer is:
gladiolus
'flower' is the definition.
(I've seen this before)
'happy soul flourished when holding single' is the wordplay.
'happy' becomes 'glad' (synonyms).
'flourished' is an anagram indicator (I've seen this in other clues).
'when holding' means one lot of letters goes inside another.
'single' becomes 'I' (one).
'soul' anagrammed gives 'olus'.
'olus' enclosing 'i' is 'iolus'.
'glad'+'iolus'='GLADIOLUS'
(Other definitions for gladiolus that I've seen before include "Flower" , "Plant of iris family" .)