Cold duck on sea current gives quack (6)
I believe the answer is:
medico
'quack' is the definition.
(I've seen this before)
'cold duck on sea current' is the wordplay.
'cold' becomes 'C' (eg on taps).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'on' says to put letters next to each other.
'sea' becomes 'med' (abbreviation of Mediterranean).
'current' becomes 'I' (symbol for electric current).
'c'+'o'='co'
'med'+'i'='medi'
'co' after 'medi' is 'MEDICO'.
'gives' is the link.
(Other definitions for medico that I've seen before include "quack" , "Doctor" , "Health practitioner" .)