Carbon monoxide starts to poison some households by old power station (3,4)
I believe the answer is:
cop shop
'station' is the definition.
(informal term for a police station)
'carbon monoxide starts to poison some households by old power' is the wordplay.
'carbon monoxide' becomes 'co' (chemical formula for carbon monoxide).
'starts to' says to take the initial letters.
'by' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'power' becomes 'p'.
The first letters of 'poison some households' is 'psh'.
'co'+'psh'+'o'+'p'='COP SHOP'
(Other definitions for cop shop that I've seen before include "Nick" , "'Police station (3,4)'" , "Police station (slang)" , "office investigating crime" .)