Pair invested in single unused stamp (7)
I believe the answer is:
imprint
'stamp' is the definition.
(I know that stamp is a more specific form of the action imprint)
'pair invested in single unused' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'invested in' is an insertion indicator.
'single' becomes 'I' (one).
'unused' becomes 'mint' (I've seen this in another clue).
'i'+'mint'='imint'
'pr' put into 'imint' is 'IMPRINT'.
(Other definitions for imprint that I've seen before include "Publisher's name on title-page of book" , "Mark produced by pressure" , "Publisher's name; stamp" , "Stamp; brand" , "Stamp; impression" .)