It is possible:
scala> implicit def iFromISD(isd: (Int, String, Double)): Int = isd._1
iFromISD: (isd: (Int, String, Double))Int
scala> val x: Int = (1, "two", 3.0)
x: Int = 1
Naturally, there has to be a type annotation on the resulting val
to drive the search for and application of the implicit conversion.
Addendum
It occurs to me there's another way that doesn't involve dubious implicit conversions:
scala> val (y, _, _) = (1, "two", 3.0)
y: Int = 1