Join retinue returning after shelling (5)

I believe the answer is:

unite

I'm a little stuck... Click here to teach me more about this clue!

'join' is the definition.
(I know that join can be written as unite)

'retinue returning after shelling' is the wordplay.
'UNITE' is hidden back-to-front in amongst the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for unite that I've seen before include "Club together" , "Get together" , "Combine - join" , "Come together as allies or comrades" , "Make one" .)

I've seen this clue in The Times.

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