Fish described by German gent in German (7)
I believe the answer is:
herring
'fish' is the definition.
(herring is a type of fish)
'german gent in german' is the wordplay.
'german gent' becomes 'herr' (I've seen this in another clue).
'german' becomes 'g' (abbreviation).
'herr'+'in'+'g'='HERRING'
'described by' is the link.
(Other definitions for herring that I've seen before include "Sprat" , "Important food fish" , "Fish that's kippered" , "Small silvery sea-fish" , "Saltwater fish" .)