Top timber contains sodium chloride (8)
I believe the answer is:
pinnacle
'top' is the definition.
(synonyms)
'timber contains sodium chloride' is the wordplay.
'timber' becomes 'pine' (I can't explain this - if you can you should give a lot more credence to this answer).
'contains' means one lot of letters goes inside another.
'sodium chloride' becomes 'nacl' (I've seen this in another clue).
'pine' enclosing 'nacl' is 'PINNACLE'.
Can you help me to learn more?
(Other definitions for pinnacle that I've seen before include "Culmination" , "The highest point, peak" , "Apogee" , "Crown" , "Highest point (of fame or building)" .)