Second letter written before, essentially describing a line that’s been crossed (7)

'that's been crossed' is the definition.
(crossbred animal)

'second letter written before essentially describing a line' is the wordplay.
'second letter written' becomes 'bee' (written name of the letter B).
'essentially' says to take the centre.
'describing' indicates putting letters inside (describe can mean to form the outline of).
'line' becomes 'l' (used when specifying particular lines from a poem).
The central letters of 'before' are 'fo'.
'a'+'l'='al'
'fo' going around 'al' is 'falo'.
'bee'+'falo'='BEEFALO'