Stamp involving small US state in a row (6)
I believe the answer is:
serial
'in a row' is the definition.
'stamp involving small us state' is the wordplay.
'stamp' becomes 'seal' (seal is a kind of stamp).
'involving' indicates putting letters inside.
'small us state' becomes 'ri' (abbreviation for Rhode Island; 'small' indicates a short form).
'seal' enclosing 'ri' is 'SERIAL'.
(Other definitions for serial that I've seen before include "Story made up of consecutive parts" , "Set of TV shows" , "following a characteristic pattern" , "Episodic story from Israel" , "In regular succession without gaps" .)