Dad goes on a ship with a free ticket (4)
I believe the answer is:
pass
'a free ticket' is the definition.
(I've seen this in another clue)
'dad goes on a ship' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'goes on' is a charade indicator (letters next to each other).
'a ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'pa'+'ss'='PASS'
'with' acts as a link.
(Other definitions for pass that I've seen before include "Proceed" , "Overtake - get through" , "Progress" , "Ratify for saps" , "way through mountains" .)