Round, almost colourless stone (4)
I believe the answer is:
opal
'stone' is the definition.
(thesaurus)
'round almost colourless' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'almost' means to remove the last letter (most of the word but not all of it).
'colourless' becomes 'pale' ('pale' can be similar in meaning to 'colourless').
'pale' with its last letter removed is 'pal'.
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "raw material" , "Birthstone for October" , "Type of gem" , "Milky iridescent stone" , "Glassy gemstone" .)