Trouble held up plant (6)
I believe the answer is:
dahlia
'plant' is the definition.
The answer is an example.
'trouble held up' is the wordplay.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'held' becomes 'had' ('have' can be a synonym of 'hold').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'ail'+'had'='ailhad'
'ailhad' in reverse letter order is 'DAHLIA'.
(Other definitions for dahlia that I've seen before include "'Hi, a lad has the flower (6)'" , "Showy garden flower" , "Showy garden plant" , "Flower, often ball-shaped" , "Had Lia this peculiar flower?" .)