Oxygen present in mostly unpolluted stream (4)
I believe the answer is:
pour
'stream' is the definition.
(synonyms)
'oxygen present in mostly unpolluted' is the wordplay.
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'present in' indicates putting letters inside.
'mostly' means to remove the last letter (most but not all of the word).
'unpolluted' becomes 'pure' (similar in meaning).
'pure' with its last letter removed is 'pur'.
'o' placed within 'pur' is 'POUR'.
(Other definitions for pour that I've seen before include "to drop, when it's heavy" , "Serve liquid" , "Teem" , "Cause to flow as from pot to cup" , "Tip liquid from container" .)