### Clue: Hard to leave playhouse Eloi built for helpful agent (3-4,3)
### Answer: TEA-TREE OIL 
 ### Explanation: Definition: helpful agent
  TEATRE["theatre"(playhouse) **minus**(to leave) "h"($$h$$ard)] + **anagram of**(built) ELOI
Defn: A natural essential oil from the leaves of a tree native to New South Wales, Australia.  A sort of multipurpose disinfectant (I can personally vouch for this), but also reported to be antiviral and antifungal.