Crossword Genius

Single family member shortly joined terror cell (4)

Ross

I believe the answer is:

isis

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'terror cell' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'single family member shortly joined' is the wordplay.
'single' becomes 'i' (one).
'family member shortly' becomes 'sis' (short for sister; 'shortly' suggests a short form).
'joined' is a charade indicator (letters next to each other).
'i'+'sis'='ISIS'

Can you help me to learn more?

(Other definitions for isis that I've seen before include "Oxford's second eight" , "Oxford river; goddess" , "academic stream?" , "Divine female" , "Egyptian goddess - Thames at Oxford" .)

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