1. [n]  a soldier who is a member of a detachment assigned to guard the flanks of a military formation
2. [n]  a back stationed wide of the scrimmage line; used as a pass receiver

FLANKER is a 7 letter word that starts with F.

1. \Flank"er\, n.
   One who, or that which, flanks, as a skirmisher or a body of
   troops sent out upon the flanks of an army toguard a line of
   march, or a fort projecting so as to command the side of an
   assailing body.
   
         They threw out flankers, and endeavored to dislodge
         their assailants.                        --W. Irwing.
   
   

2. \Flank"er\, v. t. [imp. & p. p. {Flankered}; p. pr. &
   vb. n. {Flankering}.] [See {Flank}, v. t.]
   1. To defend by lateral fortifications. [Obs.] --Sir T.
      Herbert.
   
   2. To attack sideways. [Obs.] --Evelyn.