Doctor takes on Titan, perhaps (4)

I believe the answer is:

moon

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'titan perhaps' is the definition.
I can't tell whether this definition defines the answer.

'doctor takes on' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'takes' means one lot of letters go next to another.
'mo'+'on'='MOON'

Can you help me to learn more?

(Other definitions for moon that I've seen before include "Daydream - Europa of Jupiter, say" , "Appear listless" , "Natural satellite of a planet" , "drummer?" , "orbiter" .)

I've seen this clue in The Mirror.

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