Saintly serving: a turnip with single calorie (11)
I believe the answer is:
puritanical
'saintly' is the definition.
'serving a turnip with single calorie' is the wordplay.
'serving' indicates an anagram.
'with' means one lot of letters go next to another.
'single' becomes 'i' (one).
'calorie' becomes 'cal'.
'a'+'turnip'='aturnip'
'aturnip' anagrammed gives 'puritan'.
'puritan'+'i'+'cal'='PURITANICAL'
(Other definitions for puritanical that I've seen before include "likely to curtail pleasure" , "Strictly moral in conduct" , "Very strict, censorious" , "Far from rude" , "Morally very strict" .)