Stones hit ought to be turned up (5)
I believe the answer is:
opals
'stones' is the definition.
(I've seen this before)
'hit ought to be turned up' is the wordplay.
'hit' becomes 'slap' (I've seen this before).
'ought' becomes 'o' (ought can mean zero, which resembles O).
'to be turned up' says the letters should be written in reverse.
'slap'+'o'='slapo'
'slapo' written backwards gives 'OPALS'.
(Other definitions for opals that I've seen before include "Milky gemstones with delicate colours" , "Milky white and coloured gems" , "Iridescent gems" , "Milky-white stones" , "Precious stones" .)