The planes flying could be jumbos (9)
I believe the answer is:
elephants
'be jumbos' is the definition.
(I've seen this before)
'the planes flying' is the wordplay.
'flying' is an anagram indicator.
'the'+'planes'='theplanes'
'theplanes' anagrammed gives 'ELEPHANTS'.
'could' is the link.
(Other definitions for elephants that I've seen before include "Jumbos" , "Five-toed pachyderms" , "Animals from the palest hen" , "big game" , "See 16" .)