Infant to sit outside a Post Office (7)
I believe the answer is:
papoose
'infant' is the definition.
(I know that papoose is a type of infant)
'sit outside a post office' is the wordplay.
'sit' becomes 'pose' (as in sitting for a painter).
'outside' means one lot of letters goes inside another.
'post office' becomes 'po' (abbreviation).
'a'+'po'='apo'
'pose' enclosing 'apo' is 'PAPOOSE'.
'to' is the link.
(Other definitions for papoose that I've seen before include "Pouch for carrying infant" , "American-Indian baby" , "Means to transport little one" , "Young American Indian" , "Young child" .)