Crew unknown yacht that's docked in Douglas? (4,3)
I believe the answer is:
manx cat
'that's docked in douglas?' is the definition.
(docked means with no tail)
'crew unknown yacht' is the wordplay.
'crew' becomes 'man' (I've seen this before).
'unknown' becomes 'x' (algebra).
'yacht' becomes 'cat' (short for catamaran, type of boat).
'man'+'x'+'cat'='MANX CAT'
(Other definitions for manx cat that I've seen before include "Tailless feline breed" , "Short-tailed feline" , "Domestic feline lacking a tail (4, 3)" , "Tailless animal" , "Tailless pet" .)