Why stray into boat? . . . (6)
I believe the answer is:
wherry
'boat?' is the definition.
(wherry is a kind of boat)
'why stray into' is the wordplay.
'stray' becomes 'err' ('err' can be a synonym of 'stray').
'into' means one lot of letters goes inside another.
'why' enclosing 'err' is 'WHERRY'.
(Other definitions for wherry that I've seen before include "Kind of barge" , "Shallow boat" , "Barge; small rowing-boat" , "Light rowing boat" , "Harbour rowing boat" .)