### Clue: Entertainer gets laugh changing square to circle … (4)
### Answer: HOST 
 ### Explanation: Definition: Entertainer
 "hoot"(a short outburst of laughter) **with** "S"(abbrev. for "square") **replacing**(changing … to …) "o"(letter representing a circle).
As constructed, the clue has to be read as one gets "hoot" by changing the "s" in the solution.