Frequently slip after shower, we hear, in dark period (5,2,6)
I believe the answer is:
reign of terror
'dark period' is the definition.
'Reign of Terror' can be an answer for 'period' (Reign of Terror is an example). I'm not certain of the 'dark' bit.
'frequently slip after shower we hear' is the wordplay.
'frequently' becomes 'oft' (oft can mean frequently, eg oft-quoted).
'slip' becomes 'error' (slip can mean an error or mistake).
'after' says to put letters next to each other.
'shower' becomes 'rain' (rainfall).
'we hear' shows a homophone (sound like).
'rain' sounds like 'reign'.
'oft'+'error'='ofterror'
'ofterror' after 'reign' is 'REIGN OF TERROR'.
'in' is the link.
Can you help me to learn more?
(Other definitions for reign of terror that I've seen before include "period in French history" , "French Revolutionary experience" , "Bloody period of French Revolution" , "awful time in France" , "What Robespierre started" .)