Aldermen sit around thats efficient (11)
I believe the answer is:
streamlined
'efficient' is the definition.
(similar in meaning)
'aldermen sit around' is the wordplay.
'around' indicates an anagram.
'aldermen'+'sit'='aldermensit'
'aldermensit' anagrammed gives 'STREAMLINED'.
'thats' is the link.
(Other definitions for streamlined that I've seen before include "made more efficient" , "Designed to reduce wind resistance" , "Designed for smooth flow" , "Aerodynamic" , "Shaped for speed" .)