Explanation:
what is really the definition of this f(x) ?
did you leave something out ?
the most likely answer is
f(1) = 1
as it fits many functions that fits your truncated definition.
e.g.
as you wrote it
f(x) = 1x = x
f(1) = 1
or
f(x) = 1^x
f(1) = 1¹ = 1
or
f(x) = 1/x
f(1) = 1/1 = 1
but if the real function is something else, you have to adapt this correspondingly.
f(1) means to put the value 1 into every place of x in the function definition, and then you calculate the result.
e.g. if
f(x) = 1 + x
then
f(1) = 1 + 1 = 2