Achievement I described by President, hard to follow (7)
I believe the answer is:
triumph
'achievement' is the definition.
(I have seen 'Notable achievement' mean 'triumph' so perhaps 'achievement' could also mean 'triumph')
'i described by president hard to follow' is the wordplay.
'described by' indicates putting letters inside (to describe can mean to form an outline).
'president' becomes 'trump' (Donald Trump).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to follow' means one lot of letters go next to another (some letters go after or follow others).
'i' going inside 'trump' is 'triump'.
'triump'+'h'='TRIUMPH'
(Other definitions for triumph that I've seen before include "Be victorious" , "motorbike" , "Victory, great success" , "Exult in victory" , "hit" .)