Insect starts to climb high tree (5)
I believe the answer is:
beech
'tree' is the definition.
(I know that beech is a type of tree)
'insect starts to climb high' is the wordplay.
'insect' becomes 'bee' (bee is a kind of insect).
'starts to' indicates taking the first letters.
The initial letters of 'climb high' is 'ch'.
'bee'+'ch'='BEECH'
(Other definitions for beech that I've seen before include "Common forest tree" , "Nut-bearing tree" , "Wood" , "Copper or silver tree" , "It has branches" .)