Clear line adopted by agitated vets in feed (4-7)
I believe the answer is:
'clear' is the definition.
'line adopted by agitated vets in feed' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'agitated' indicates anagramming the letters.
'vetsinfeed' is an anagram of 'sefevident'.
'l' put into 'sefevident' is 'SELF-EVIDENT'.
(Other definitions for self-evident that I've seen before include "Not needing to be explained" , "Obvious" , "Needing no explanation" , "Not requiring further explanation" , "Not needing to be demonstrated" .)