Stone is best - start to finish! (5)
I believe the answer is:
topaz
'stone' is the definition.
(I've seen this before)
'best start to finish' is the wordplay.
'best' becomes 'tops'.
'start' means to remove the last letter (I've seen 'starts' mean this).
'to finish' becomes 'az' (I can't explain this - if you can you should believe this answer much more).
'tops' with its last letter removed is 'top'.
'top'+'az'='TOPAZ'
'is' is the link.
Can you help me to learn more?
(Other definitions for topaz that I've seen before include "South American hummingbird" , "Silicate gemstone" , "White or colourless gemstone" , "Precious stone - American hummingbird" , "Yellow or pale blue gem" .)