Gemstone set my hat off (8)
I believe the answer is:
amethyst
'gemstone' is the definition.
(amethyst is a kind of gemstone)
'set my hat off' is the wordplay.
'off' indicates anagramming the letters.
'set'+'my'+'hat'='setmyhat'
'setmyhat' is an anagram of 'AMETHYST'.
(Other definitions for amethyst that I've seen before include "February stone" , "Semiprecious stone of violet or purple quartz" , "Violet gemstone" , "Precious stone of purple quartz" , "Purple quartz used in jewellery" .)