They say there's space for a head-cold (5)

I believe the answer is:

rheum

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'cold' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for rheum that I've seen before include "Poetic tears" , "Eyes watering" , "Rhubarb genus; a head cold" , "Flowing water" , "Watery fluid" .)

I've seen this clue in the Sydney Morning Herald.

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