Feline's caught in joint up one old tree (9)
I believe the answer is:
pistachio
'tree' is the definition.
(pistachio tree is a kind of tree)
'feline's caught in joint up one old' is the wordplay.
'feline's' becomes 'cat's' (cat is a kind of feline).
'caught in' indicates putting letters inside.
'joint' becomes 'hip joint' (part of body).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'one old' becomes 'io' (I am not sure about this - if you are sure you should believe this answer much more).
'cats' put inside 'hip' is 'hcatsip'.
'hcatsip' in reverse letter order is 'pistach'.
'pistach'+'io'='PISTACHIO'
Can you help me to learn more?
(Other definitions for pistachio that I've seen before include "Greenish nut" , "Edible pale green seed" , "nuts" , "Yellowish-green" , "Food" .)