Ale-drinking achievement by English yeoman (9)
I believe the answer is:
beefeater
'yeoman' is the definition.
(I know that yeoman can be written as beefeater)
'ale-drinking achievement by english' is the wordplay.
'ale' becomes 'beer' (I've seen this in other clues).
'drinking' means one lot of letters goes inside another (inserted letters are consumed or drunk).
'achievement' becomes 'feat' (feat is a kind of achievement).
'by' says to put letters next to each other.
'english' becomes 'e' (abbreviation).
'feat' put after 'e' is 'efeat'.
'beer' placed around 'efeat' is 'BEEFEATER'.
(Other definitions for beefeater that I've seen before include "no vegetarian?" , "Bodyguard of the British monarch" , "Yeoman warder of the Tower of London" , "One might guard" , "Royal servant" .)