Explosive to look good on casual outfit (5,4)

I believe the answer is:

shell suit

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'casual outfit' is the definition.
'shell suit' can be an answer for 'outfit' (I've seen this before). I am not sure about the 'casual' bit.

'explosive to look good on' is the wordplay.
'explosive' becomes 'shell' (I can't justify this - if you can you should give a lot more credence to this answer).
'to look good on' becomes 'suit' (I've seen this in another clue).
'shell'+'suit'='SHELL-SUIT'

Can you help me to learn more?

(Another definition for shell suit that I've seen is " Outfit".)

I've seen this clue in The Daily Express.

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