React to a sign that something needs including (5)
I believe the answer is:
caret
'sign that something needs including' is the definition.
'caret' can be an answer for 'sign' (caret is a kind of sign). I'm unsure of the remainder of the definition.
'react' is the wordplay.
'react' can be anagrammed to 'CARET'.
But, I'm not clear how the anagram is indicated.
'to a' acts as a link.
Can you help me to learn more?
(Other definitions for caret that I've seen before include "Particular mark" , "Text mark indicating insertion" , "this reveals something missing" , "Indicator of omission" , "Weeding tool" .)