Occultism threatening astrologers, complacent at first (5,5)
I believe the answer is:
black magic
'occultism' is the definition.
'threatening astrologers complacent at first' is the wordplay.
'threatening' becomes 'black' (black can mean foreboding).
'astrologers' becomes 'magi'.
'at first' says to take the initial letters.
The initial letter of 'complacent' is 'c'.
'black'+'magi'+'c'='BLACK MAGIC'
(Other definitions for black magic that I've seen before include "Evil sorcery" , "bad spelling?" , "Evil use of supernatural power" , "Wizardry for evil purposes" , "The dark arts" .)