Carbon monoxide poisoning's beginning to permeate numerous soldiers (7)
I believe the answer is:
company
'soldiers' is the definition.
(company can mean a group of soldiers)
'carbon monoxide poisoning's beginning to permeate numerous' is the wordplay.
'carbon monoxide' becomes 'CO' (chemical formula for carbon monoxide).
'beginning' says to take the initial letters.
'to permeate' indicates putting letters inside (permeate means to go into or fill).
'numerous' becomes 'many' (similar in meaning).
The first letter of 'poisonings' is 'p'.
'p' placed within 'many' is 'mpany'.
'co'+'mpany'='COMPANY'
(Other definitions for company that I've seen before include "Firm, concern" , "Commercial business" , "See 10" , "Group of soldiers, business" , "Crew" .)