Is allowed to put the tuber back (3)
I believe the answer is:
may
'is allowed to' is the definition.
(I've seen this in another clue)
'the tuber back' is the wordplay.
'the tuber' becomes 'yam' (yam is a kind of tuber).
'back' shows that the letters should be reversed in order.
'yam' reversed gives 'MAY'.
'put' acts as a link.
(Other definitions for may that I've seen before include "Fifth month" , "maid" , "Time" , "Might" , "Month - tree" .)