Tentatively schedule antibiotic I left out (in different places) (6,2)

I believe the answer is:
pencil in
'tentatively schedule antibiotic i left' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'in different places' is the wordplay.
I cannot quite understand how this works, but
'in' is found in the answer.
The remaining letters 'pencil' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'out' is the link.
Can you help me to learn more?
(Other definitions for pencil in that I've seen before include "Provisionally schedule" , "Provisionally reserve" .)
