Better sort of dash back — joiner’s outside (5)
I believe the answer is:
amend
'better' is the definition.
('amend' can be a synonym of 'better')
'sort of dash back joiner's outside' is the wordplay.
'sort of dash' becomes 'em' (em dash, typographical term).
'back' says the letters should be written in reverse.
'joiner' becomes 'and' (I've seen this before).
'outside' means one lot of letters goes inside another.
'em' in reverse letter order is 'me'.
'me' put into 'and' is 'AMEND'.
(Other definitions for amend that I've seen before include "Alter in writing" , "Make minor improvements to (something)" , "Modify - edit" , "Change to improve" , "Make small improvements to" .)