Err seriously at first, dividing award for informal musicmaking (8)
I believe the answer is:
singsong
'informal musicmaking' is the definition.
'err seriously at first dividing award' is the wordplay.
'err' becomes 'sin' (I've seen this before).
'at first' suggests taking the first letters.
'dividing' indicates putting letters inside.
'award' becomes 'gong' (I've seen this before**).
The initial letter of 'seriously' is 's'.
's' placed inside 'gong' is 'gsong'.
'sin'+'gsong'='SINGSONG'
'for' is the link.
(Other definitions for singsong that I've seen before include "musical get-together" , "With a rise and fall of voice in speaking" , "group recital" , "Informal choral session" , "Friendly musical session" .)