Brand leader in trouble - there's a letter about it (6)
I believe the answer is:
Here is my best explanation:
'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 "Mark of shame, stain on reputation", "sign of disgrace", "Bad mark", "Mark of dishonour", "A mark of shame", "Symbol of disgrace or infamy", "Brand or mark of infamy", "A stain as on character", "Mark of shame or social disgrace" and "evidence of nastiness?".)