Joining transport heading off on motorway (8)
I believe the answer is:
marriage
'joining' is the definition.
(I've seen this in another clue)
'transport heading off on motorway' is the wordplay.
'transport' becomes 'carriage' (being carried or transported).
'heading off' means to remove the first letter.
'on' is a charade indicator (letters next to each other).
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'carriage' with its initial letter taken away is 'arriage'.
'arriage' after 'm' is 'MARRIAGE'.
(Other definitions for marriage that I've seen before include ""Till death do us part" arrangement" , "Church occasion" , "Wedlock of alliance" , "nuptials" , "The nuptial ceremony" .)