Pet fish died: bowl terrible, ultimately (6)
I believe the answer is:
coddle
'pet' is the definition.
(thesaurus)
'fish died bowl terrible ultimately' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'ultimately' indicates one should take the final letters.
The last letters of 'died bowl terrible' are 'dle'.
'cod'+'dle'='CODDLE'
(Other definitions for coddle that I've seen before include "Cook in nearly-boiling water" , "Traditional Dublin dish of boiled sausages etc" , "Spoil, pamper" , "Pamper with traditional Dublin food dish" , "Cook (eggs) in hot water - pamper" .)