Master with an angle in front of massive vortex (9)
I believe the answer is:
maelstrom
'vortex' is the definition.
(I know that vortex can be written as maelstrom)
'master with an angle in front of massive' is the wordplay.
'master' becomes 'maestro' (synonyms).
'with' is an insertion indicator.
'an angle' becomes 'l'.
'in front of' says to take the initial letters.
The initial letter of 'massive' is 'm'.
'maestro' going around 'l' is 'maelstro'.
'maelstro'+'m'='MAELSTROM'
(Other definitions for maelstrom that I've seen before include "disorder" , "Powerful circular current of water" , "Strong circular current of water" , "Confused situation" , "violent upheaval" .)