Ending on motorways, hundred sheep get lost! (5)
I believe the answer is:
scram
'get lost' is the definition.
(thesaurus)
'ending on motorways hundred sheep' is the wordplay.
'ending on' indicates one should take the final letters.
'hundred' becomes 'c' (Roman numeral).
'sheep' becomes 'ram' (ram is a kind of sheep**).
The final letter of 'motorways' is 's'.
's'+'c'+'ram'='SCRAM'
(Other definitions for scram that I've seen before include "Go away quickly" , "Scarper" , "Bolt" , "Clear off!" , "Skedaddle!" .)