Flowers once used to punish offenders (6)
I believe the answer is:
stocks
'flowers' is the definition.
(stock is a kind of flower)
'used to punish offenders' is the wordplay.
I cannot really see how this works, but
'to' is found in the answer.
This explanation may well be incorrect...
'once' is the link.
Can you help me to learn more?
(Other definitions for stocks that I've seen before include "Fragrant flowers; shares" , "In which miscreant is locked" , "Capital holdings often accompanied by shares" , "Plants; form of punishment" , "Keeps" .)