Second employer of rodent exterminator (6)
I believe the answer is:
mouser
'rodent exterminator' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'second employer' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'employer' becomes 'user' (someone who employs or uses something).
'mo'+'user'='MOUSER'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for mouser that I've seen before include "Cat as hunter of small rodents" , "Predatory cat" , "Cat proficient at catching small rodents" , "moggy" , "Rodent catcher" .)