Brand leader in trouble - there's a letter about it (6)
I believe the answer is:
'brand' is the definition.
('stigma' can be a synonym of 'brand')
'leader in trouble there's a letter about it' is the wordplay.
'leader in' suggests taking the first letters.
'there's a letter' becomes 'sigma' (sigma is a kind of letter**).
'about it' means one lot of letters goes inside another.
The initial letter of 'trouble' is 't'.
't' going inside 'sigma' is 'STIGMA'.
(Other definitions for stigma that I've seen before include "Disgraceful mark" , "Mark of infamy or disgrace" , "Blot" , "Mark of shame or social disgrace" , "Sign of disgrace" .)