Pair invested in one mint stamp (7)
I believe the answer is:
imprint
'stamp' is the definition.
(I know this)
'pair invested in one mint' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'invested in' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'i'+'mint'='imint'
'pr' inserted inside 'imint' is 'IMPRINT'.
(Other definitions for imprint that I've seen before include "Stamp; impression" , "Stamp; brand" , "Mark produced by pressure" , "Stamp (with sense of identity)" , "Mark made by pressing or stamping surface" .)