Which version of Lisp are you talking about? A very brief web search showed eg https://stackoverflow.com/questions/40210896/syntax-error-in...

For something really simple, try `(define define)` in Scheme.

I'd say Forth has less syntax than Lisp. But that's perhaps a reflection of most Forth implementations using some clever tricks to offload much of what's handled by syntax in other languages.

Eg defining new functions (ie 'words' in Forth terms) is not done via a special syntactic construct in Forth, but via 'immediate words'. The lines are a but blurry, though.

It isn't. It's just a valid S-expr.

For example

((a b c) (d e f)) is not a valid Common Lisp program.

(let foo bar baz) is also not a valid Common Lisp program.

The syntax for LET is:

   let ({var | (var [init-form])}*) declaration* form*

  ; in: LET A
  ;     (LET A
  ;       B
  ;       C)
  ; 
  ; caught ERROR:
  ;   Malformed LET bindings: A.
  ; 
  ; compilation unit finished
  ;   caught 1 ERROR condition