Actually, calling it an F6 without the root (1st or prime) could make sense, or a Dm7 without the minor third. In either case, if you're in the key of C, or have modulated to the key of C temporarily, it is performing the function of the subdominant. F6 and Dm7 are different names for the same combination of notes. F is the subdominant of C and Dm is it's parallel minor.

This chord may actually not be truncated, depending on what's in the melody, and it may just have been "broken", i.e., the root may have been played before or after this (partial) chord. In cases like this, one has to check the immediate area around the chord. It also helps to know what key one is in at a given moment, which is not necessarily what the key signature shows (i.e., you might have modulated).