Foolish time to start to climb up tree (7)
I believe the answer is:
apricot
'tree' is the definition.
(apricot is a kind of tree)
'foolish time to start to climb up' is the wordplay.
'foolish time' becomes 'apri' (I can't justify this - if you can you should believe this answer much more).
'start to' indicates taking the first letters.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
The first letter of 'climb' is 'c'.
'to'+'c'='toc'
'toc' in reverse letter order is 'cot'.
'apri'+'cot'='APRICOT'
Can you help me to learn more?
(Other definitions for apricot that I've seen before include "Orange-yellow fruit in plum family" , "Asian fruit" , "Fruit from a tropic" , "Juicy fruit" , "Juicy stone-fruit related to plum and peach" .)