Send Tristan the wrong way round motorway (8)
I believe the answer is:
Here is my best explanation:
'send' is the definition.
('transmit' can be a synonym of 'send')
'tristan the wrong way round motorway' is the wordplay.
'the wrong way' indicates anagramming the letters (I've seen this in other clues).
'round' is an insertion indicator.
'motorway' becomes 'm'.
'tristan' is an anagram of 'transit'.
'transit' enclosing 'm' is 'TRANSMIT'.