Lexicographer, originally from OU, dispatched entire contents of Chambers? (7)
I believe the answer is:
roomful
'entire contents of chambers?' is the definition.
(contents of a room)
'lexicographer originally from ou dispatched' is the wordplay.
'originally' suggests taking the first letters.
'dispatched' is an anagram indicator (letters dispatched to new positions).
The initial letter of 'lexicographer' is 'l'.
'l'+'from'+'ou'='lfromou'
'lfromou' with letters rearranged gives 'ROOMFUL'.
(Other definitions for roomful that I've seen before include "Enough to fill a domestic space" , "A lot of people" , "library's volume, say" , "As many as will fit in a domestic space" , "wall-to-wall assembly?" .)