Touch a slow boat on the way back (4)
I believe the answer is:
abut
'touch' is the definition.
(abutting is a kind of touching)
'a slow boat on the way back' is the wordplay.
'slow boat' becomes 'tub' (I've seen this before).
'on the way back' says the letters should be written backwards.
'tub' reversed gives 'but'.
'a'+'but'='ABUT'
(Other definitions for abut that I've seen before include "Impinge" , "Touch (side of next building)" , "Neighbour" , "Adjoin at one end" , "Adjoin -- 19 (rev)" .)