A month to work on sea creature (7)
I believe the answer is:
octopus
'sea creature' is the definition.
(I've seen this in another clue)
'a month to work on' is the wordplay.
'a month' becomes 'Oct' (abbreviation for October).
'to work' becomes 'opus' (an opus is an artistic work).
'on' means one lot of letters go next to another.
'oct'+'opus'='OCTOPUS'
(Other definitions for octopus that I've seen before include "Eight-armed creature" , "one that's heavily armed?" , "Eight-limbed mollusc" , "Sea-creature" , "army swimmer" .)