Powerful organisation needing work after month (7)
I believe the answer is:
octopus
'powerful organisation' is the definition.
(octopus can mean a sinister organisation)
'work after month' is the wordplay.
'work' becomes 'opus' (an opus is an artistic work).
'after' means one lot of letters go next to another.
'month' becomes 'oct' (abbreviation for October).
'opus' after 'oct' is 'OCTOPUS'.
'needing' acts as a link.
(Other definitions for octopus that I've seen before include "Tentacled sea creature" , "Mollusc with long tentacles" , "Eight-legged creature" , "Tentacled marine mollusc" , "Sea creature - pulpo on a Spanish menu" .)