Source : Webster's Revised Unabridged Dictionary (1913)

Clinker \Clink"er\ (kl[i^][ng]k"[~e]r), n. [From clink; cf. D.
   clinker a brick which is so hard that it makes a sonorous
   sound, from clinken to clink. Cf. {Clinkstone}.]
   1. A mass composed of several bricks run together by the
      action of the fire in the kiln.

   2. Scoria or vitrified incombustible matter, formed in a
      grate or furnace where anthracite coal in used; vitrified
      or burnt matter ejected from a volcano; slag.

   3. A scale of oxide of iron, formed in forging.

   4. A kind of brick. See {Dutch clinker}, under {Dutch}.

Source : WordNet®

clinker
     n 1: a fragment of incombustible matter left after a wood or coal
          or charcoal fire [syn: {cinder}]
     2: a hard brick used as a paving stone [syn: {clinker brick}]
     v 1: clear out the cinders and clinker from; "we clinkered the
          fire frequently"
     2: turn to clinker or form clinker under excessive heat in
        burning