Crossword Genius

Lord Jim agonised, getting some sexually mature insect (5)


I believe the answer is:

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

'lord jim agonised getting some sexually' is the wordplay.
'IMAGO' is hidden in amongst the letters.
However, I'm uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for imago that I've seen before include "Mature insect after metamorphosis", "That's as far as a butterfly can go", "spinner, say", "latest development in cricket?", "Last stage of adult insect".)

I've seen this clue in Sydney Morning Herald.