Promise to start work on the highway (4)
I believe the answer is:
word
'promise' is the definition.
(as in 'I give you my word')
'start work on the highway' is the wordplay.
'start' indicates taking the first letters.
'the highway' becomes 'rd'.
The first letters of 'work on' is 'wo'.
'wo'+'rd'='WORD'
'to' acts as a link.
(Other definitions for word that I've seen before include "Promise - unit of language" , "Remark; news" , "Message" , "News; speech element" , "Communication; news" .)