Part of aircraft also involved motion (8)
I believe the answer is:
proposal
'motion' is the definition.
(I've seen this before)
'part of aircraft also involved' is the wordplay.
'part of aircraft' becomes 'prop' (short for propeller).
'involved' indicates an anagram (involved can mean confusing or complex).
'also' anagrammed gives 'osal'.
'prop'+'osal'='PROPOSAL'
(Other definitions for proposal that I've seen before include "Offer - suggestion" , "a match might be arranged?" , "A suggestion, perhaps of marriage" , "union offer?" , "Suggestion put forward for consideration" .)