Flower trouble kept returning (6)
I believe the answer is:
dahlia
'flower' is the definition.
(dahlia is a kind of flower)
'trouble kept returning' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'kept' becomes 'had' (I've seen this in another clue).
'returning' is a reversal indicator.
'ail'+'had'='ailhad'
'ailhad' written backwards gives 'DAHLIA'.
(Other definitions for dahlia that I've seen before include "Flower with pompom variety" , "Brightly coloured garden flower with tuberous roots" , "Showy garden plant" , "Ali had (anag) - showy flower" , "Plant with tuberous roots and showy coloured flower heads" .)