Best mate heads off to find stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'best mate heads off' is the wordplay.
'best' becomes 'op' (I can't justify this - if you can you should believe this answer much more).
'mate' becomes 'pal' (I've seen this in another clue).
'heads off' means to remove the first letter.
'pal' with its initial letter taken off is 'al'.
'op'+'al'='OPAL'
'to find' acts as a link.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "Type of gem" , "Milky-white gem" , "Mineral used as a gemstone, of hydrated silica" , "it may be highly valued" , "Mineral, usually iridescent" .)