No lager drunk by northern chap (8)
I believe the answer is:
algernon
'chap' is the definition.
'no lager drunk by northern' is the wordplay.
'drunk' indicates an anagram.
'by' means one lot of letters go next to another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'lager' is an anagram of 'alger'.
'no' put after 'alger' is 'algerno'.
'algerno'+'n'='ALGERNON'
(Other definitions for algernon that I've seen before include "- - Moncrieff (Wilde)" , "Boy" , "Nag loner (anag) ___ boy's name" , "lad" , "-- Swinburne, English poet" .)