Center letter: c
Outer letter: a d i t u v
Answers (20):
  viaduct
  acacia
  acai
  acid
  acidic
  addict
  adduct
  attic
  cacti
  cava
  cicada
  civic
  dicta
  didact
  didactic
  ducat
  duct
  tacit
  tact
  tactic