Rev about to appreciate something valuable (6)
I believe the answer is:
nugget
'something valuable' is the definition.
(I've seen this before)
'rev about to appreciate' is the wordplay.
'rev' becomes 'gun' (I've seen this in another clue).
'about' says the letters should be written in reverse.
'to appreciate' becomes 'get' (I've seen this in another clue).
'gun' written backwards gives 'nug'.
'nug'+'get'='NUGGET'
(Other definitions for nugget that I've seen before include "Solid lump of a precious metal" , "Piece of precious metal (or chicken?)" , "Small lump (of something valuable)" , "Lump of gold in natural state" , "Lump of valuable metal" .)