Against observers using houses (6)
I believe the answer is:
versus
'against' is the definition.
(thesaurus)
'observers using houses' is the wordplay.
'houses' indicates the answer is hidden within the clue (part of the clue houses the answer).
'VERSUS' can be found hidden inside 'observers using'.
(Other definitions for versus that I've seen before include "taking part in team sports" , "In opposition to" , "v. (sport)" , "Set against" , "playing" .)