Gas that is to escape slowly after houseroom evacuated (8)
I believe the answer is:
schmooze
'gas' is the definition.
(both can mean to chat)
'that is to escape slowly after houseroom evacuated' is the wordplay.
'that is' becomes 'sc' (abbrv. for Latin scilicet; 'namely').
'to escape slowly' becomes 'ooze'.
'after' says to put letters next to each other.
'evacuated' says to hollow out the word (remove centre letters).
'houseroom' with its centre taken out is 'hm'.
'ooze' put after 'hm' is 'hmooze'.
'sc'+'hmooze'='SCHMOOZE'
(Other definitions for schmooze that I've seen before include "Chat at a gathering" , "Talk amicably, perhaps seeking a favour" , "Chat in friendly manner" , "Chat - gossip" , "Chat socially" .)