True when at sea with navy a volley possibly needed (6)
I believe the answer is:
return
'a volley possibly needed' is the definition.
(tennis)
'true when at sea with navy' is the wordplay.
'when at sea' is an anagram indicator (at sea can mean in confusion or chaos).
'with' means one lot of letters go next to another.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'true' is an anagram of 'retu'.
'retu'+'rn'='RETURN'
(Other definitions for return that I've seen before include "hit back?" , "Reappear" , "Profit earned on money" , "Come/go back" , "reply" .)