Hair treatment that's bad, at some stage? (5)
I believe the answer is:
singe
'hair treatment' is the definition.
(I've seen this in another clue)
'bad at some stage?' is the wordplay.
'bad' becomes 'sin'.
'at some stage?' becomes 'ge' (I can't explain this - if you can you should believe this answer much more).
'sin'+'ge'='SINGE'
'that's' is the link.
Can you help me to learn more?
(Other definitions for singe that I've seen before include "Burn slightly on surface" , "Char, scorch" , "Scorch lightly" , "Result of burning" , "slight burn" .)