Rodent catcher spends little time for employer (4)
I believe the answer is:
user
'employer' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for user that I've seen before include "junkie" , "Drug taker" , "Patron" , "Utiliser" , "One who exploits others" .)