Crossword Genius

Decoration to book suggesting safe investment? (4,4)

Ross

I believe the answer is:

gilt edge

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

'decoration to book suggesting safe' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'investment?' is the wordplay.
I cannot quite understand how this works, but
'investment?' could be 'gilt' (type of UK fixed-interest security) and 'gilt' is found within the answer.
The remaining letters 'edge' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Globe and Mail.
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