Menacing trendy relative outside (8)
I believe the answer is:
sinister
'menacing' is the definition.
(I know that menacing can be written as sinister)
'trendy relative outside' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'relative' becomes 'sister' (sister is a kind of relative).
'outside' means one lot of letters goes inside another (some letters go outside others).
'in' going into 'sister' is 'SINISTER'.
(Other definitions for sinister that I've seen before include "Suggestive of evil, to the left in heraldry" , "On the left (of shield) (her.); wicked, criminal" , "Threatening the left in heraldry" , "Suggestive of evil, being lefthanded" , "not right!" .)