Aimed crayon, terribly streamlined (11)
I believe the answer is:
aerodynamic
'streamlined' is the definition.
(streamlined for travel through air)
'aimed crayon terribly' is the wordplay.
'terribly' indicates an anagram.
'aimed'+'crayon'='aimedcrayon'
'aimedcrayon' with letters rearranged gives 'AERODYNAMIC'.
(Other definitions for aerodynamic that I've seen before include "could be made to fly?" , "Designed like a bird" , "Streamlined" , "Fit to be a high-flyer" , "very good on air?" .)