Admiral quizzes initially unruly Utica marine (7)
I believe the answer is:
aquatic
'marine' is the definition.
(similar in meaning)
'admiral quizzes initially unruly utica' is the wordplay.
'initially' indicates taking the first letters.
'unruly' is an anagram indicator.
The initial letters of 'admiral quizzes' is 'aq'.
'utica' anagrammed gives 'uatic'.
'aq'+'uatic'='AQUATIC'
(Other definitions for aquatic that I've seen before include "Of the water, its animals, its sports" , "Marine - living in water" , "Growing or living in water" , "Growing in or near water" , "At home in water" .)