May return small tubers (4)
I believe the answer is:
yams
'tubers' is the definition.
(yam is a kind of tuber)
'may return small' is the wordplay.
'return' says the letters should be written in reverse.
'small' becomes 's' (abbreviation - e.g. clothes size).
'may' written backwards gives 'yam'.
'yam'+'s'='YAMS'
(Other definitions for yams that I've seen before include "Edible tubers from the tropics" , "Sweet potatoes" , "Roots" , "vegetables" .)