Spot a tortoise eating a tuber (6)
I believe the answer is:
potato
'a tuber' is the definition.
(I've seen this before)
'spot a tortoise eating' is the wordplay.
'eating' indicates a hidden word.
'POTATO' can be found hidden inside 'spot a tortoise'.
(Other definitions for potato that I've seen before include "The staple food is strangely too apt" , "Starchy vegetable" , "Inert person" , "E.g. King Edward" , "Tuber, popular in Ireland" .)