1) just define function as usual:
  {def add  {lambda {:a :b :c} {+ :a :b :c}}} -> add

2) and use it:
  {add 1 2 3}     -> 6
  {{add 1} 2 3}   -> 6
  {{add 1 2} 3}   -> 6
  {{{add 1} 2} 3} -> 6

3) application:
{def fs {lambda {:f} map :f}}
{def f1 {lambda {:x} {* :x 2}}}
{def f2 {lambda {:x} {pow :x 2}}}
{def fsf1 {fs f1}}
{def fsf2 {fs f2}}

{{fsf1} 0 1 2 3}
{{fsf2} 0 1 2 3}
{{fsf1} 2 4 6 8}
{{fsf2} 2 4 6 8}

Output:
0 2 4 6
0 1 4 9
4 8 12 16
4 16 36 64