Lacking doctor, worker on oil rig becomes sicker (5)

I believe the answer is:

iller

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'sicker' is the definition.
('ill' can be similar in meaning to 'sick')

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

(Other definitions for iller that I've seen before include "Even more 9" , "Less well" , "In worse health" .)

I've seen this clue in The Hindu.

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