Vessel that sinks but buoyant occasionally (1-4)
I believe the answer is:
u-boat
'vessel that sinks' is the definition.
'u boat' can be an answer for 'vessel' (I've seen this before). I'm not certain of the rest of the definition.
'but buoyant occasionally' is the wordplay.
'occasionally' indicates alternate letters.
'but'+'buoyant'='butbuoyant'
The alternating letters of 'butbuoyant' are 'U-BOAT'.
Can you help me to learn more?
(Other definitions for u-boat that I've seen before include "German wartime submarine" , "German WW 2 sub" , "German submarine of WW2" , "Take pool exercise" , "German World War One or World War Two submarine" .)