Newton, brought in to obtain fish, is polite (7)
I believe the answer is:
'is polite' is the definition.
'newton brought in to obtain fish' is the wordplay.
'newton' becomes 'N' (synonyms).
'brought in to' is an insertion indicator.
'obtain' becomes 'get' (synonyms).
'fish' becomes 'eel' (eel is a kind of fish).
'n' put into 'get' is 'gent'.
(Other definitions for genteel that I've seen before include "Cultured; polite" , ""Refayned"" , "Exaggeratedly polite" , "Polite and well-bred" , "Well mannered" .)