Warning of short period without cover (4)
I believe the answer is:
omen
'warning' is the definition.
(omen can mean a warning or sign of the future)
'short period without cover' is the wordplay.
'short' becomes 'ome' (I can't justify this - if you can you should give a lot more credence to this answer).
'period' becomes 'morning ' (a period of the day).
'without cover' says to take the centre (the letters which cover up the word are removed).
The middle letter of 'morning' is 'n'.
'ome'+'n'='OMEN'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for omen that I've seen before include "writing on the wall" , "Prognostication" , "Portent, prophetic indicator" , "Sign of likely future event" , "Augury, portent" .)