Sodium chloride in tree top (8)
I believe the answer is:
pinnacle
'top' is the definition.
(I know that top can be written as pinnacle)
'sodium chloride in tree' is the wordplay.
'sodium chloride' becomes 'nacl' (I've seen this in another clue).
'in' is an insertion indicator.
'tree' becomes 'pine' (pine tree is a kind of tree).
'nacl' going within 'pine' is 'PINNACLE'.
(Other definitions for pinnacle that I've seen before include "Culmination" , "The highest point, summit" , "Acme" , "Apogee" , "The highest point, peak" .)