Book that is about design for aircraft (7)
I believe the answer is:
biplane
'aircraft' is the definition.
(biplane is a kind of aircraft)
'book that is about design' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'about' means one lot of letters goes inside another.
'design' becomes 'plan' (synonyms).
'ie' placed around 'plan' is 'iplane'.
'b'+'iplane'='BIPLANE'
'for' acts as a link.
(Other definitions for biplane that I've seen before include "craft" , "Aircraft with double wings" , "Old aircraft" , "flying machine" , "Early aircraft" .)