Wellness lacking originally in old PM (6)
I believe the answer is:
health
'wellness' is the definition.
(I've seen this in another clue)
'lacking originally in old pm' is the wordplay.
'originally' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'old pm' becomes 'Heath' (prime minister Edward Heath).
The initial letter of 'lacking' is 'l'.
'l' placed inside 'heath' is 'HEALTH'.
(Other definitions for health that I've seen before include "constitution" , "10 18's great creation" , "Fine condition" , "State of wellness" , "Physical welfare" .)