1. [n]  someone who fits a garment to a particular person
2. [adj]  improved in health or physical condition

FITTER is a 6 letter word that starts with F.

1. \Fit"ter\, n.
   1. One who fits or makes to fit; esp.:
      (a) One who tries on, and adjusts, articles of dress.
      (b) One who fits or adjusts the different parts of
          machinery to each other.
   
   2. A coal broker who conducts the sales between the owner of
      a coal pit and the shipper. [Eng.] --Simmonds.
   
   

2. \Fit"ter\, n.
   A little piece; a flitter; a flinder. [Obs.]
   
         Where's the Frenchman? Alas, he's all fitters. --Beau.
                                                  & Fl.