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

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "clinker":
   adobe, alluvion, alluvium, ash, ashes, ashlar, atonalism,
   atonality, brand, brick, bricks and mortar, cacophony, calx,
   carbon, cement, charcoal, cinder, coal, coke, concrete, coom,
   covering materials, deposition, deposits, diluvium, discord,
   discordance, discordancy, disharmony, dissonance, dissonancy,
   dissonant chord, draff, dregs, dross, ember, feces, ferroconcrete,
   firebrick, flag, flagstone, flatness, flooring, froth, fume,
   grounds, inharmoniousness, inharmony, lath and plaster, lava, lees,
   loess, masonry, moraine, mortar, off note, offscum, pavement,
   paving, paving material, plasters, precipitate, precipitation,
   prestressed concrete, reek, roofage, roofing, scoria, scum,
   sediment, settlings, sharpness, siding, silt, sinter, slag, smoke,
   smudge, smut, soot, sour note, sourness, stone, stridor, sublimate,
   sullage, tile, tiling, tunelessness, unharmoniousness,
   unmusicality, untunefulness, walling, wolf