A zealot initially going after finest gemstone (5)
I believe the answer is:
topaz
'gemstone' is the definition.
(topaz is a kind of gemstone)
'a zealot initially going after finest' is the wordplay.
'initially' says to take the initial letters.
'going after' says to put letters next to each other.
'finest' becomes 'top' (I've seen this before).
The initial letter of 'zealot' is 'z'.
'a'+'z'='az'
'az' after 'top' is 'TOPAZ'.
(Other definitions for topaz that I've seen before include "Type of gemstone" , "Silicate gemstone" , "Precious stone - American hummingbird" , "Semi-precious stone of various colours, especially yellow" , "Precious stone - dark yellow colour" .)