Stipulation that visor's taken off inside post office (7)
I believe the answer is:
proviso
'stipulation' is the definition.
(I know that proviso is a type of stipulation)
'visor's taken off inside post office' is the wordplay.
I cannot really understand how this works, but
'post' could be 'p' (abbreviation as in PS for postscript) and 'p' is present in the answer.
an anagram of 'visor' is 'rovis' which is located in the answer.
A single letter 'o' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'that' acts as a link.
Can you help me to learn more?
(Other definitions for proviso that I've seen before include "Something insisted upon as part of an agreement" , "Condition added to an agreement" , "Condition for VIP or so" , "Rider" , "Stipulation (in contract)" .)