Cost of sending item with label in place (7)
I believe the answer is:
postage
'cost of sending item' is the definition.
(cost of posting something)
'label in place' is the wordplay.
'label' becomes 'tag' (tag is a kind of label).
'in' means one lot of letters goes inside another.
'place' becomes 'pose' (posing is a kind of placing).
'tag' placed within 'pose' is 'POSTAGE'.
'with' acts as a link.
(Other definitions for postage that I've seen before include "Amount paid to mail" , "Cost to send a mail item" , "Delivery cost" , "Mailing cost" , "It could be first-class" .)