Crossword Genius

Two last in line prompted to go up (5)

Ross

I believe the answer is:

deuce

'two' is the definition.
('deuce' can be a synonym of 'two')

'last in line prompted to go up' is the wordplay.
'last in' says to take the final letters.
'prompted' becomes 'cued' ('cue' can be a synonym of 'prompt').
'to go up' is a reversal indicator (I've seen 'goes up' mean this).
The final letter of 'line' is 'e'.
'e'+'cued'='ecued'
'ecued' in reverse letter order is 'DEUCE'.

(Other definitions for deuce that I've seen before include "situation of no advantage to either side" , "with equal score" , "Devil" , "score at Wimbledon?" , "Level of tennis" .)

I've seen this clue in The Times.
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