Cross empty highway and propose to maintain run (6)
I believe the answer is:
hybrid
'cross' is the definition.
(I've seen this in another clue)
'empty highway and propose to maintain run' is the wordplay.
'empty' means to remove the middle letters.
'and' means one lot of letters go next to another.
'propose' becomes 'bid'.
'to maintain' indicates putting letters inside (maintain can mean to keep or preserve).
'run' becomes 'r' (cricket abbreviation).
'highway' with its centre removed is 'hy'.
'bid' placed around 'r' is 'brid'.
'hy'+'brid'='HYBRID'
(Other definitions for hybrid that I've seen before include "Genetic cross" , "Cross of two different species or varieties" , "Compound, mixture" , "Plant that is the offspring of two different species" , "Of mixed character" .)