I'm not sure your explanation is clear enough.
If there is a number followed by a letter (a - d) in the 2nd position in the list, do you want to remove BOTH the number and the letter? Or just the number, leaving the letter?
Do you want to do that for EVERY item in the list? Or just the pair in the 2nd position?
Using your example, what should be the result?
$class = 7c,6a,9,8b < remove the whole 2nd item ("9a")
$class = 7c,a,6a,9,8b < remove just the number from the 2nd item ("9a" becomes "a")
$class = c,a,a,9,b < remove each number that precedes a letter -- but don't alter an item that just consists of a single number
$class = 7c,6a,9,8b < remove the "9a" from the 2nd position if another "9" (without a letter) is found somewhere else in the list ("9" is just arbitrary -- any matching number will do)