Recognizing Horn clauses
Let's start with some definitions:


A literal is either a variable (a positive literal) or the negation of a variable (a negative literal).
A clause is a formula which is equivalent to a disjunction of literals.
A Horn clause is a formula which is equivalent to a disjunction of literals, at most one of which is positive.


Your formula $(P \to Q) \to W$ is not equivalent to a disjunction of literals. Indeed, if it were, since it depends on all variables $P,Q,W$, it would have exactly one falsifying assignment (with respect to $P,Q,W$). Yet it has three: $(P,Q,W)=(F,F,F),(F,T,F),(T,T,F)$. So it is not a clause, and in particular not a Horn clause.