Taxpayers update piggy-bank contents when salary arrives (6)
I believe the answer is:
payday
'taxpayers update piggy-bank contents' is the definition.
I can't judge whether this definition defines the answer.
'salary arrives' is the wordplay.
'salary' becomes 'pay' (synonyms).
'arrives' becomes 'day' (I am not sure about this - if you are sure you should believe this answer much more).
'pay'+'day'='PAYDAY'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for payday that I've seen before include "Time when wages are received" , "when wages are given" .)