Private planes not primarily for flying (8)
I believe the answer is:
personal
'private' is the definition.
(associated in meaning)
'planes not primarily for flying' is the wordplay.
'not primarily' means to remove the first letter.
'flying' is an anagram indicator (letters fly around into new positions).
'for' with its initial letter removed is 'or'.
'planes'+'or'='planesor'
'planesor' anagrammed gives 'PERSONAL'.
(Other definitions for personal that I've seen before include "Distinctive; individual" , "Belonging to an individual" , "Pertaining to the self" , "Individual; advert (column)" , "Such a rude remark" .)