The most common and most likely answer to this crossword clue is the 7 letter word FAIRING
F
A
I
R
I
N
G
Buddy explains!
The straight part
We know that "What’ll aid streamlining" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’ll aid streamlining = fairing.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FAIRING mean?
FAIRING verb- To smoothen or even a surface (especially a connection or junction on a surface).
- To bring into perfect alignment (especially about rivet holes when connecting structural members).
- To construct or design a structure whose primary function is to produce a smooth outline or reduce air drag or water resistance.
- To make fair or beautiful.
- A structure on various parts of a vehicle, for example an aircraft, automobile, or motorcycle, that produces a smooth exterior and reduces drag
- A present; originally, one given or purchased at a fair.
- Something edible; fare.