The one playing outside is making a din (5)
I believe the answer is:
noise
'a din' is the definition.
(a din is a noisy disturbance)
'the one playing outside is' is the wordplay.
'playing' indicates anagramming the letters.
'outside' indicates putting letters inside.
'one' is an anagram of 'noe'.
'noe' placed around 'is' is 'NOISE'.
'making' acts as a link.
(Other definitions for noise that I've seen before include "Uproar" , "Unstructured sound" , "Clatter" , "Loud or unpleasant sound" , "Racket, din" .)