Greek character in boat easily training Athenian beginners (4)
I believe the answer is:
beta
'greek character' is the definition.
(letter from the Greek alphabet)
'boat easily training athenian beginners' is the wordplay.
'beginners' says to take the initial letters.
The first letters of 'boat easily training athenian' is 'BETA'.
'in' is the link.
(Other definitions for beta that I've seen before include "Second-class" , "Second-rate" , "final test" , "Old character" , "Character between alpha and gamma" .)