One facing op may have to take this by mouth (3)
I believe the answer is:
nil
This clue could be a double definition.
'one' is the first definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'facing op may have to take this by mouth' is the second definition.
The definition suggests a singular noun which matches the answer.
(Other definitions for nil that I've seen before include "Non-existent" , "Nix, nada, nothing" , "Nothing, zero" , "Nothing, naught" , "You get nothing" .)