Eight-armed mollusc (7)
I believe the answer is:
octopus
'eight-armed mollusc' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for octopus that I've seen before include "Invertebrate sea creature with three hearts" , "person with widespread influence" , "army swimmer" , "Sea creature - pulpo on a Spanish menu" , "large, influential organization" .)