Cocoa used in navy for mammal (7)
I believe the answer is:
raccoon
'for mammal' is the definition.
(raccoon is a kind of mammal)
'cocoa used in navy' is the wordplay.
'used' indicates an anagram.
'in' means one lot of letters goes inside another.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'cocoa' is an anagram of 'accoo'.
'accoo' going inside 'rn' is 'RACCOON'.
(Other definitions for raccoon that I've seen before include "Greyish-brown American mammal" , "Furry grey forest mammal, black-banded on face and tail" , "North American omnivore" , "Omnivorous nocturnal mammal native to N and C America" , "Smallish mammal" .)