Order for one sailor to make a U-turn (3-2)
I believe the answer is:
say-so
'order' is the definition.
(I've seen this before)
'for one sailor to make a u-turn' is the wordplay.
'for one' becomes 'say' (both can mean 'for example').
'sailor' becomes 'os' (ordinary seaman).
'to make a u-turn' says the letters should be written backwards (letters turn around).
'os' in reverse letter order is 'so'.
'say'+'so'='SAY-SO'
(Other definitions for say-so that I've seen before include "Go-ahead" , "Authority to make a decision" , "Arbitrary assertion" , "Authority - rumour" , "Final approval" .)