You can only have one at a sitting (7)
I believe the answer is:
session
'a sitting' is the definition.
(eg a parliamentary session)
'you can only have one at' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'at' could be 'on' (location) and 'on' is found within the answer.
The remaining letters 'sess' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for session that I've seen before include "Period devoted to an activity" , "Meeting devoted to a particular purpose" , "Time in legislature" , "Studio recording period" , "Meeting or series of meetings" .)