Dangerous fish: it’s angry when disturbed (8)
I believe the answer is:
stingray
'dangerous fish' is the definition.
(I've seen this before)
'it's angry when disturbed' is the wordplay.
'when disturbed' indicates anagramming the letters.
'its'+'angry'='itsangry'
'itsangry' anagrammed gives 'STINGRAY'.
(Other definitions for stingray that I've seen before include "Large venomous fish -- it's angry (anag)" , "Marine creature with venomous tail" , "Poisonous spined fish" , "swimmer" , "menace in the water" .)