What a grind when this was operating under sail (8)
I believe the answer is:
windmill
'this was operating under sail' is the definition.
The definition suggests a singular noun which matches the answer.
'what a grind' is the wordplay.
'what' becomes 'wind' (I can't justify this - if you can you should believe this answer much more).
'a grind' becomes 'mill' (synonyms).
'wind'+'mill'='WINDMILL'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for windmill that I've seen before include "Natural energy generator" , "Apparatus driven by moving air" , "Power generator with sails" , "It forms energy from moving air" , "Building for grinding grain" .)