Introduce one with stern disposition (6)
I believe the answer is:
insert
'introduce' is the definition.
(I know that introduce can be written as insert)
'one with stern disposition' is the wordplay.
'one' becomes 'i' (Roman numeral).
'with' means one lot of letters go next to another.
'disposition' is an anagram indicator.
'stern' anagrammed gives 'nsert'.
'i'+'nsert'='INSERT'
(Other definitions for insert that I've seen before include "Introduce, put in" , "Add in" , "Place within, pages of a publication say" , "One might be seen" , "Push into" .)