Where you might find porters plausibly missing? I'll say! (3)
I believe the answer is:
pub
'where you might find porters plausibly missing? i'll say' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
This is all the clue.
Can you help me to learn more?
(Other definitions for pub that I've seen before include "A familiar hostelry" , "Tavern, hostelry" , "Slug and Lettuce, perhaps" , "Alehouse" , "free house?" .)