Great wave means trouble in front of the ship (6)
I believe the answer is:
billow
'great wave' is the definition.
(I've seen this before)
'trouble in front of the ship' is the wordplay.
'trouble' becomes 'ill' ('ill' can be a synonym of 'trouble').
'in' indicates putting letters inside.
'front of the ship' becomes 'bow'.
'ill' inserted into 'bow' is 'BILLOW'.
'means' is the link.
(Other definitions for billow that I've seen before include "Swell out (as a sail)" , "Wave; upward flow" , "Swell out in the wind" , "A great wave or swell" , "Rise or roll like wave" .)