Most tight locks save energy close to aqueduct (7)
I believe the answer is:
meanest
'most tight' is the definition.
(tight and mean can both mean stingy with money)
'close to aqueduct' is the wordplay.
'close' becomes 'mean' (I've seen this before).
'to aqueduct' becomes 'est' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mean'+'est'='MEANEST'
'locks save energy' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for meanest that I've seen before include "Most cruel" , "Most stingy, like 16" , "of least value" , "Most miserable" , "Most unpleasant" .)