Crossword Genius

Leading measure when considering steam? (5,2)

Ross

I believe the answer is:

ahead of

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

'leading' is the definition.
(I've seen this in another clue)

'measure when considering steam?' is the wordplay.
I cannot quite see how this works, but
'measure' could be 'head' (head is a kind of measure) and 'head' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Guardian.
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