Infant in pose outside a Post Office (7)
I believe the answer is:
papoose
'infant' is the definition.
(papoose is a kind of infant)
'pose outside a post office' is the wordplay.
'outside' means one lot of letters goes inside another.
'post office' becomes 'po'.
'a'+'po'='apo'
'pose' placed around 'apo' is 'PAPOOSE'.
'in' acts as a link.
(Other definitions for papoose that I've seen before include "Young child" , "American-Indian baby" , "A Red Indian baby" , "Young American Indian" , "Pouch for carrying infant" .)