Old secretary put on pounds or stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'old secretary put on pounds' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'secretary' becomes 'PA' (abbreviation for personal assistant).
'put on' is a charade indicator (letters next to each other).
'pounds' becomes 'L' (L can represent the monetary pound).
'o'+'pa'+'l'='OPAL'
'or' acts as a link.
(Other definitions for opal that I've seen before include "item in Hatton Garden?" , "Shimmering stone" , "Translucent gem" , "Iridescent quartz-like stone" , "Gem with delicate colours" .)