In the outhouse? Not I! (4)
I believe the answer is:
thou
'i' is the 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?
'in the outhouse? not' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THOU' is hidden in the letters of 'outhouse not'.
Can you help me to learn more?
(Other definitions for thou that I've seen before include "Old-fashioned pronoun" , "You (archaic, but still used in prayers)" , "Biblical 'you'" , "You (arch.)" , "You, poetically of old" .)