Crossword Genius

Soldiers losing second off-road bike (4)

Ross

I believe the answer is:

quad

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

'bike' is the definition.
(I've seen this before)

'soldiers losing second off-road' is the wordplay.
I cannot quite understand how this works, but
'second' could be 'd' and 'd' is found within the answer.
The remaining letters 'qua' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for quad that I've seen before include "space found in building" , "Child of four" , "One of four offspring at the same birth (abbr)" , "Enclosed courtyard" , "Sound < playground" .)

I've seen this clue in The Times.
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