Employ co-host, on and off, for certain periods (6)
I believe the answer is:
epochs
'certain periods' is the definition.
'epochs' can be an answer for 'periods' (I have seen 'Historical periods ' mean 'epochs' so perhaps 'periods' could also mean 'epochs'). I'm not sure about the 'certain' bit.
'employ co-host on and off' is the wordplay.
'on and off' means one should take alternating letters (letters selected on and off at regular intervals).
'employ'+'co'+'host'='employcohost'
The alternate letters of 'employcohost' are 'EPOCHS'.
'for' is the link.
Can you help me to learn more?
(Other definitions for epochs that I've seen before include "Geological ages" , "Eras, subdivisions of time" , "Vast periods of time" , "Remarkable period of time" , "Particular periods of time" .)