The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
      [1913 Webster]

   3. A scale of oxide of iron, formed in forging.
      [1913 Webster]

   4. A kind of brick. See Dutch clinker, under Dutch.
      [1913 Webster]

WordNet (r) 3.0 (2006):

clinker
    n 1: a fragment of incombustible matter left after a wood or
         coal or charcoal fire [syn: cinder, clinker]
    2: a hard brick used as a paving stone [syn: clinker, 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