Second pen containing large stone (8)
I believe the answer is:
sparkler
'stone' is the definition.
(sparkler can refer to a gem)
'second pen containing large' is the wordplay.
'second' becomes 's' (common abbreviation).
'pen' becomes 'parker' (Parker is a pen manufacturer).
'containing' indicates putting letters inside.
'large' becomes 'l'.
'parker' enclosing 'l' is 'parkler'.
's'+'parkler'='SPARKLER'
(Other definitions for sparkler that I've seen before include "Bright gem" , "Eg, a diamond" , "Possibly brilliant" , "Jewel" , "Firework or diamond ring" .)