Flowering plant’s trouble: kept regressing (6)
I believe the answer is:
dahlia
'flowering plant's' is the definition.
(dahlia is a kind of flowering plant)
'trouble kept regressing' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'kept' becomes 'had' (I've seen this in another clue).
'regressing' says the letters should be written in reverse (to regress is to go backwards).
'ail'+'had'='ailhad'
'ailhad' backwards is 'DAHLIA'.
(Other definitions for dahlia that I've seen before include "Aid Hal (anag) - flower with showy heads" , "Had Lia this peculiar flower?" , "Strangely, Ali had this flower" , "one in bed" , "'Hi, a lad has the flower (6)'" .)