Closing period? (4,4)
I believe the answer is:
full stop
'closing period?' is the definition.
'full stop' can be an answer for 'period?' ('full stop' can be a synonym of 'period'). I am not sure about the 'closing' bit.
'closing period?' is the wordplay.
'closing' becomes 'fulls' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'period?' becomes 'top' (top is a kind of period).
'fulls'+'top'='FULL STOP'
Can you help me to learn more?
(Other definitions for full stop that I've seen before include "Punctuation mark - period" , "A period in the US!" , "'Punctuation mark (4,4)'" , "Point" , "period in America" .)