Cow producing pure nitrogen! (7)
I believe the answer is:
chasten
'cow' is the definition.
(both can mean to subdue or humble)
'pure nitrogen' is the wordplay.
'pure' becomes 'chaste' (chaste means abstaining from sex).
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'chaste'+'n'='CHASTEN'
'producing' is the link.
(Other definitions for chasten that I've seen before include "Punish, purify" , "Castigate or punish" , "Censure severely" , "Subdue - a stench (anag)" , "Subdue pride of with a stench" .)