A current at sea (6)
I believe the answer is:
adrift
'at sea' is the definition.
(I've seen this before)
'a current' is the wordplay.
'current' becomes 'drift' (can be similar in meaning in navigation contexts).
'a'+'drift'='ADRIFT'
(Other definitions for adrift that I've seen before include "Unanchored at sea" , "getting nowhere?" , "Moving aimlessly, unfastened" , "Loose from moorings" , "Afloat without steerage" .)