Thanks FemmeNoire,
15a - Yes, it is a foreign word.
35a - Is a Japanese electronics manufacturer of, among other things, "keyboard" which goes around a theme word - definition is "British sprinter" (he specialised as a 400m relay runner)
51a - Anagram/"demolished" of "oak tree n(ot) + theme word - definition is "great stone faced film star".
49d - Is hidden in the clue - definition is "money"
There is no 23a - did you mean 23d? 6 letter "promotes" (as in 'lifts up') + 4 letter "second man" (from the Bible) - definition is "causes trouble"