Rear Admiral tucking into mean fish (8)
I believe the answer is:
stingray
'fish' is the definition.
(type of fish)
'rear admiral tucking into mean' is the wordplay.
'rear admiral' becomes 'RA' (naval abbreviation).
'tucking into' means one lot of letters goes inside another.
'mean' becomes 'stingy' (both can mean miserly).
'ra' inserted into 'stingy' is 'STINGRAY'.
(Other definitions for stingray that I've seen before include "Large venomous fish -- it's angry (anag)" , "Dangerous fish" , "Marine creature with venomous tail" , "Fish with venomous tail" , "Poisonous spined fish" .)