REPINE is a 6 letter word that starts with R.

1. \Re*pine"\ (r?-p?n"), v. i. [Pref. re- + pine to
   languish.]
   1. To fail; to wane. [Obs.] ``Reppening courage yields no
      foot to foe.'' --Spenser.
   
   2. To continue pining; to feel inward discontent which preys
      on the spirits; to indulge in envy or complaint; to
      murmur.
   
            But Lachesis thereat gan to repine.   --Spenser.
   
            What if the head, the eye, or ear repined To serve
            mere engines to the ruling mind?      --Pope.
   
   

2. \Re*pine"\, n.
   Vexation; mortification. [Obs.] --Shak.