Cautionary signal shown by lamb erroneously (5)
I believe the answer is:
amber
'cautionary signal' is the definition.
(I've seen this before)
'shown by lamb erroneously' is the wordplay.
'shown by' indicates a hidden word.
'AMBER' is hidden in the letters of 'lamb erroneously'.
(Other definitions for amber that I've seen before include "Neither red nor green?" , "Traffic-light colour" , "gives warning" , "Honey-yellow colour" , "Yellowish fossilised resin" .)