Speed at sea in shark: no, tuna (4)
I believe the answer is:
knot
'speed at sea' is the definition.
(measure of speed once used on ships)
'in shark no tuna' is the wordplay.
'in' indicates a hidden word.
'KNOT' is hidden within 'shark no tuna'.
(Other definitions for knot that I've seen before include "Fastening formed by tying string etc" , "Ring < sandpiper" , "Tie (or what one puts in it?)" , "Small sandpiper - fastening device" , "Small cluster - huddled group - sheepshank, for example" .)