### Clue: Crew unknown yacht that's docked in Douglas? (4,3)
### Answer: MANX CAT 
 ### Explanation: Definition: that's docked in Douglas
 MAN(to crew/to staff/to fill empty positions in an organisation, say) + X(symbol for an unknown quantity in mathematics) + CAT(short for "catamaran"/a yacht with twin hulls in parallel).
Defn: "That"/a breed of cats with a naturally docked/short or non-existent tail, originally from the Isle of Man, whose capital is Douglas.