Underground trapper's a hero hoarding books (7)

I believe the answer is:

antlion

'underground trapper's' is the definition.
(insect which traps ants)

'a hero hoarding books' is the wordplay.
'hero' becomes 'lion' (lion can mean a brave person).
'hoarding' indicates putting letters inside.
'books' becomes 'NT' (New Testament).
'a'+'lion'='alion'
'alion' enclosing 'nt' is 'ANTLION'.

(Another definition for antlion that I've seen is " Insect".)

I've seen this clue in The Times.

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