A king, in dread, unable to sleep? (5)
I believe the answer is:
awake
'unable to sleep?' is the definition.
(laying awake)
'a king in dread' is the wordplay.
'king' becomes 'k' (abbreviation for king in chess).
'in' means one lot of letters goes inside another.
'dread' becomes 'awe' (awe can mean a feeling of dread).
'a'+'k'='ak'
'ak' inserted into 'awe' is 'AWAKE'.
(Other definitions for awake that I've seen before include "Conscious, aware" , "Alive and alert" , "Make active again" , "Patriotic song 'The West's .....'" , "Cease sleeping" .)