Uncle had a sleep coming back in the boat (6)
I believe the answer is:
sampan
'boat' is the definition.
(sampan is a kind of boat)
'uncle had a sleep coming back' is the wordplay.
'uncle' becomes 'sam' (Uncle Sam is an example).
'had' means one lot of letters go next to another.
'a sleep' becomes 'nap' ('nap' can be a synonym of 'sleep').
'coming back' is a reversal indicator.
'nap' in reverse letter order is 'pan'.
'sam'+'pan'='SAMPAN'
'in the' acts as a link.
(Other definitions for sampan that I've seen before include "oriental vessel" , "Small 10 boat" , "Asian skiff" , "Boat in Far East" , "Small flat-bottomed boat in China" .)