Going back a bit, recall a poster showing one of the Stones (4)
I believe the answer is:
opal
'one of the stones' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'going back a bit recall a poster' is the wordplay.
'going back' says the letters should be written in reverse.
'a bit' indicates a hidden word (the answer is formed by a bit of the clue).
'lapo' can be found hidden inside 'recall a poster'.
'lapo' backwards is 'OPAL'.
'showing' is the link.
Can you help me to learn more?
(Other definitions for opal that I've seen before include "it may be highly valued" , "Type of gemstone" , "Mineral, usually iridescent" , "Gem stone" , "Something milky-white" .)