Gun dog disturbs a mammal (6)
I believe the answer is:
dugong
'a mammal' is the definition.
(dugong is a kind of mammal)
'gun dog disturbs' is the wordplay.
'gun dog' becomes 'gundog' (synonyms).
'disturbs' indicates anagramming the letters (I've seen 'disturbed' mean this (the order of letters is disturbed)).
'gundog' with letters rearranged gives 'DUGONG'.
(Other definitions for dugong that I've seen before include "Sea cow - gun dog (anag)" , "Marine creature" , "Marine mammal" , "Sea beast" , "Sea cow with forked tail" .)