Stole M Sartre's case in waterside shed (9)
I believe the answer is:
boathouse
'shed' is the definition.
(I know that boathouse is a type of shed)
'stole m sartre's case' is the wordplay.
'stole' becomes 'boa' (both can mean a type of scarf).
'm' becomes 'thou' (synonyms).
'case' says to hollow out the word (remove centre letters).
'sartre' with its middle removed is 'se'.
'boa'+'thou'+'se'='BOATHOUSE'
'in waterside' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for boathouse that I've seen before include "building by water's edge" , "Riverside construction" , "Riverside structure" , "Bank building" .)