Fish of the most important kind I start to collect - in aspiration (10)
I believe the answer is:
idealistic
'fish of the most important kind i start to collect in aspiration' is the definition.
I can't judge whether this defines the answer.
'fish of the most important kind i start to collect' is the wordplay.
'fish of the most important kind' becomes 'idealist' (I can't explain this - if you can you should believe this answer much more).
'start to' indicates taking the first letters.
The first letter of 'collect' is 'c'.
'idealist'+'i'+'c'='IDEALISTIC'
Can you help me to learn more?
(Other definitions for idealistic that I've seen before include "Naively impractical" , "High-minded - italicised (anag)" , "Visionary" , "Impracticable" , "Seeking the best" .)