Deviation at sea from way back (3)
I believe the answer is:
yaw
'deviation at sea' is the definition.
(I've seen this in another clue)
'way back' is the wordplay.
'back' says the letters should be written backwards.
'way' written backwards gives 'YAW'.
'from' acts as a link.
(Other definitions for yaw that I've seen before include "Go off course" , "(Of ship) twist sideways" , "Deviate off course" , "Deviation (of e.g. ship)" , "Twisting motion (of ship)" .)