Cross empty highway and start to breathe free (6)
I believe the answer is:
hybrid
'cross' is the definition.
(I've seen this before)
'empty highway and start to breathe free' is the wordplay.
'empty' means to remove the middle letters.
'and' means one lot of letters go next to another.
'start to' says to take the initial letters.
'free' becomes 'rid' (similar in meaning).
'highway' with its middle taken out is 'hy'.
The first letter of 'breathe' is 'b'.
'hy'+'b'+'rid'='HYBRID'
(Other definitions for hybrid that I've seen before include "Plant that is the offspring of two different species" , "Crossbred" , "Of mixed ancestry" , "Genetic cross" , "Like a mule" .)