Horse swallowing a very soft stone (8)
I believe the answer is:
sapphire
'stone' is the definition.
(I know that sapphire is a type of transparent gem)
'horse swallowing a very soft' is the wordplay.
'horse' becomes 'shire horse' (shire horse is a kind of horse).
'swallowing' is an insertion indicator.
'very soft' becomes 'pp' (musical abbreviation for pianissimo).
'a'+'pp'='app'
'shire' placed around 'app' is 'SAPPHIRE'.
(Other definitions for sapphire that I've seen before include "His paper can be turned into a jewel" , "A gem; I, perhaps (anag.)" , "Blue gemstone" , "His paper can be made into a jewel" , "Precious stone of rich blue" .)