Periodically sang and instrument chimed (6)
I believe the answer is:
agreed
'chimed' is the definition.
'periodically sang and instrument' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'and' says to put letters next to each other.
'instrument' becomes 'reed' (reed is a kind of instrument).
The alternate letters of 'sang' are 'ag'.
'ag'+'reed'='AGREED'
(Other definitions for agreed that I've seen before include "In harmony" , "Saw eye to eye" , "Accorded" , "Consented and concurred" , "I'll go along with that" .)