### Clue: Second letter written before, essentially describing a line that's been crossed (7)
### Answer: BEEFALO 
 ### Explanation: Definition: that's been crossed
definition: a hybrid / 'cross' between cows and bison/buffalo 
BEE (B, the "Second letter" in the alphabet) + FO (the centre/essential part of be-FO-re) around A + L (line)