Emerald isn't terribly aerodynamic (11)
I believe the answer is:
streamlined
'aerodynamic' is the definition.
(I know that aerodynamic can be written as streamlined)
'emerald isn't terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'emerald'+'isnt'='emeraldisnt'
'emeraldisnt' is an anagram of 'STREAMLINED'.
(Other definitions for streamlined that I've seen before include "Shaped for speed" , "made more efficient" , "Designed for smooth flow" , "Designed to reduce wind resistance" , "Of sleek appearance" .)