In case, when rough, there are no boats (6)
I believe the answer is:
canoes
'boats' is the definition.
(canoe is a kind of boat)
'in case when rough there are no' is the wordplay.
I cannot quite see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found in the answer.
'no' could be 'o' (O resembles 0) and 'o' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for canoes that I've seen before include "Craft" , "Small boats found in strange oceans" , "Simple boats" , "Small light boats" .)