Crossword Genius

What may help you make slam or blast? (4,5)

Ross

I believe the answer is:

last trump

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

'what may help you make slam' is the definition.
I can't judge whether this defines the answer.

'blast?' is the wordplay.
I cannot really see how this works, but
'blast?' could be 'trump' (I've seen this before) and 'trump' is found in the answer.
The remaining letters 'last' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'or' is the link.

Can you help me to learn more?

I've seen this clue in The Times.
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