Old leader of orchestra is boring, less threatening (7)
I believe the answer is:
ominous
'threatening' is the definition.
(synonyms)
'old leader of orchestra is boring less' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'leader of' says to take the initial letters.
'is boring' means one lot of letters goes inside another (some letters must 'bore' a hole into other letters).
'less' becomes 'minus' (I've seen this before).
The initial letter of 'orchestra' is 'o'.
'o' put within 'minus' is 'minous'.
'o'+'minous'='OMINOUS'
(Other definitions for ominous that I've seen before include "Sinister-looking" , "Threatening, looks bad" , "Foreboding, menacing" , "Threatening, portending tragedy" , "Foreboding, threatening" .)