Ultimately turned on, perhaps, excited crustacean (4,6)
I believe the answer is:
sand hopper
'crustacean' is the definition.
(I've seen this before)
'ultimately turned on perhaps excited' is the wordplay.
'ultimately' says to take the final letters.
'excited' is an anagram indicator.
The final letter of 'turned' is 'd'.
'd'+'on'+'perhaps'='donperhaps'
'donperhaps' anagrammed gives 'SAND-HOPPER'.
(Other definitions for sand hopper that I've seen before include "jumper by the seaside?" , "Small beach crustacean" .)