Worker starts to climb highest tree (5)
I believe the answer is:
beech
'tree' is the definition.
(beech tree is a kind of tree)
'worker starts to climb highest' is the wordplay.
'worker' becomes 'bee' (I've seen this before).
'starts to' says to take the initial letters.
The first letters of 'climb highest' is 'ch'.
'bee'+'ch'='BEECH'
(Other definitions for beech that I've seen before include "Timber tree" , "Tree of Fagus genus" , "Tree variety" , "Nut-bearing tree" , "Deciduous tree with smooth grey bark" .)