One's blooming trouble had returned (6)
I believe the answer is:
dahlia
'one's blooming' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'trouble had returned' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'returned' is a reversal indicator.
'ail'+'had'='ailhad'
'ailhad' back-to-front is 'DAHLIA'.
Can you help me to learn more?
(Other definitions for dahlia that I've seen before include "Composite" , "conspicuous bloomer" , "Aid Hal (anag) - flower with showy heads" , "one in bed" , "Ali had (anag) - showy flower" .)