Pressure initially over North Dakota’s source of water (4)
I believe the answer is:
pond
'water' is the definition.
(I know that pond is a type of water)
'pressure initially over north dakota's source' is the wordplay.
'initially' says to take the initial letters.
'source' suggests taking the first letters (source can mean a beginning or origin).
The first letter of 'pressure' is 'p'.
The initial letters of 'over north dakotas' is 'ond'.
'p'+'ond'='POND'
'of' acts as a link.
(Other definitions for pond that I've seen before include "Small lake" , "Small area of still water" , "Pool of still water" , "Small body of water" , "Himalayan country" .)