Plant in trouble had up (6)
I believe the answer is:
dahlia
'plant in' is the definition.
'dahlia' can be an answer for 'plant' (I've seen this before). I am not sure about the 'in' bit.
'trouble had up' is the wordplay.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ail'+'had'='ailhad'
'ailhad' written backwards gives 'DAHLIA'.
Can you help me to learn more?
(Other definitions for dahlia that I've seen before include "Suggestive of soil" , "one in bed" , "Plant with tuberous roots and showy coloured flower heads" , "Garden plant with large brightly-coloured flowers" , "Flower, often ball-shaped" .)