Horned god rising in sleep (3)
I believe the answer is:
nap
'sleep' is the definition.
(a nap is a short sleep)
'horned god rising' is the wordplay.
'horned god' becomes 'pan' (I've seen this in another clue).
'rising' says the letters should be written in reverse (going up in a down clue).
'pan' written backwards gives 'NAP'.
'in' acts as a link.
(Other definitions for nap that I've seen before include "Raised surface of a fabric and short sleep" , "Card game - quick sleep" , "Sleep - card game - fibres on a snooker table" , "Siesta" , "A short day-sleep" .)