A worry, having to stuff something in a clue? (7)
I believe the answer is:
anagram
'something in a clue?' is the definition.
(cryptic crossword clues often use anagrams)
'a worry having to stuff' is the wordplay.
'worry' becomes 'nag' (nagging is a kind of worrying).
'having' means one lot of letters go next to another.
'to stuff' becomes 'ram' (ram can mean to stuff or push).
'a'+'nag'+'ram'='ANAGRAM'
(Other definitions for anagram that I've seen before include "Word that rearranges another's letters" , "Word rearranging another's letters" , "Word arranged from the letters of another" , "pouring the tea?" , "Argue the point" .)