Starchy tubers may upset stomach initially (4)
I believe the answer is:
yams
'starchy tubers' is the definition.
(I've seen this in another clue)
'may upset stomach initially' is the wordplay.
'upset' says the letters should be written in reverse.
'initially' suggests taking the first letters.
The initial letter of 'stomach' is 's'.
'may' reversed gives 'yam'.
'yam'+'s'='YAMS'
(Other definitions for yams that I've seen before include "vegetables" , "Roots" , "Sweet potatoes" , "Edible tubers from the tropics" .)