The Collaborative International Dictionary of English v.0.48:

Unclog \Un*clog"\, v. t. [1st pref. un- + clog.]
   To disencumber of a clog, or of difficulties and
   obstructions; to free from encumbrances; to set at liberty.
   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unclog
    v 1: become or cause to become unobstructed; "The chemical that
         we poured down the drain unclogged it" [ant: back up,
         choke, choke off, clog, clog up, congest, foul]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "unclog":
   aid, bare, blow, blow out, clean out, clear, clear away, clear off,
   clear out, clear the decks, clear the way, defecate, deobstruct,
   deplete, disclose, drain, ease, eliminate, empty, empty out,
   evacuate, exhaust, expedite, explain, expose, facilitate, free,
   grease, grease the ways, grease the wheels, hasten, help along,
   loose, lubricate, make clear, make way for, manifest, oil,
   open the way, open up, patefy, pave the way, prepare the way,
   purge, quicken, remove, remove friction, reveal,
   run interference for, scour out, simplify, smooth, smooth the way,
   soap the ways, speed, sweep out, unbar, unblock, uncase, unclench,
   unclutch, uncork, uncover, uncurtain, undo, undrape, unfold,
   unfoul, unjam, unlatch, unlock, unplug, unroll, unseal, unsheathe,
   unshut, unstop, unveil, unwrap, vent, void