Rest after busy period (4,4)
I believe the answer is:
full stop
'period' is the definition.
(I know that period can be written as full stop)
'rest after busy' is the wordplay.
'rest' becomes 'stop'.
'after' means one lot of letters go next to another.
'busy' becomes 'full'.
'stop' put after 'full' is 'FULL-STOP'.
(Other definitions for full stop that I've seen before include "Punctuation mark at end of sentence" , "Point" , "finalised" , "Punctuation mark - period" , "Period of sentence" .)