Relatives locked up by Trinidad security (4)

I believe the answer is:

dads

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

'relatives' is the definition.
(dad is a kind of relative)

'locked up by trinidad security' is the wordplay.
'DADS' is hidden within the letters.
But, I'm unsure how this is indicated.

Can you help me to learn more?

(Other definitions for dads that I've seen before include "Family men" , "Fathers, informally" , "1 across 10" , "Fathers, in short" .)

I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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