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 anagramming the letters (involved can mean confusing or complex).
'also' is an anagram of '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" .)