Strict dietician gave off nitrogen (5)
I believe the answer is:
vegan
'strict dietician' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'gave off nitrogen' is the wordplay.
'off' indicates an anagram.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'gave' is an anagram of 'vega'.
'vega'+'n'='VEGAN'
Can you help me to learn more?
(Other definitions for vegan that I've seen before include "one not using animals" , "Non-consumer of animal products" , "Avoiding animal products" , "Conscientious consumer" , "One not eating animals" .)