Over-sleeps allotted time (4)
I believe the answer is:
span
'allotted time' is the definition.
'span' can be an answer for 'time' (thesaurus). I'm unsure of the 'allotted' bit.
'over-sleeps' is the wordplay.
'over' says the letters should be written backwards.
'sleeps' becomes 'naps' ('nap' can be a synonym of 'sleep').
'naps' written backwards gives 'SPAN'.
Can you help me to learn more?
(Other definitions for span that I've seen before include "Bridge - rotated" , "Extent from end to end or across" , "Extent, space for odd pans" , "Pair of horses" , "Length of bridge - 9 inches!" .)