One is distracted by northern light (4)
I believe the answer is:
neon
'light' is the definition.
'one is distracted by northern' is the wordplay.
'is distracted' is an anagram indicator.
'by' says to put letters next to each other.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'one' anagrammed gives 'neo'.
'neo'+'n'='NEON'
(Other definitions for neon that I've seen before include "Type of lamp" , "None like bright city light" , "One of the six inert gases" , "Atomic number 10" , "It may illuminate" .)