Naval officer, one with two daughters in motor yacht (5)
I believe the answer is:
middy
'naval officer' is the definition.
(short midshipman)
'one with two daughters in motor yacht' is the wordplay.
'one' becomes 'i' (roman numeral).
'with' is a charade indicator (letters next to each other).
'two daughters' becomes 'dd' (I've seen this in other clues. I am not sure about the 'two' bit.).
'in' is an insertion indicator.
'motor yacht' becomes 'my' (abbreviation for motor yacht).
'i'+'dd'='idd'
'idd' going within 'my' is 'MIDDY'.
(Other definitions for middy that I've seen before include "boy to be seen aboard" , "Sailor" .)