Can't comment on the flight deck procedures, but, technically, an aircraft is cleared for take-off when ATC have issued the clearance, the crew have read it back, and ATC have not corrected any part of the readback. For a professional pilot to have any doubts about this would be worrying.
In practise I've never had any problems with engines increasing power as the clearance is read back, but one needs to be aware of predictive or confirmation bias creeping in to such situations.