Under water, it takes a month to work! (7)
I believe the answer is:
octopus
'under water' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'a month to work' is the wordplay.
'a month' becomes 'Oct' (abbreviation for October).
'to work' becomes 'opus' (an opus is an artistic work).
'oct'+'opus'='OCTOPUS'
'it takes' is the link.
Can you help me to learn more?
(Other definitions for octopus that I've seen before include "Marine creature" , "Tentacled, shell-less mollusc" , "one finds, in the main" , "Sea creature with tentacles" , "Influential person" .)