A short dash (6)
I believe the answer is:
hyphen
'a short dash' is the definition.
'dash' is the wordplay.
I cannot really see how this works, but
'dash' could be 'en' (an en dash is a dash as wide as a letter 'n') and 'en' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for hyphen that I've seen before include "Sign joining words" , "A horizontal word joiner" , "Word-joining mark" , "Line in a double-barrelled name, say" , "Horizontal words joiner" .)