Motion forward, also deviating (8)
I believe the answer is:
Here is my best explanation:
'motion' is the definition.
'forward also deviating' is the subsidiary indication.
'forward' becomes 'prop'.
'deviating' is an anagram indicator.
'also' with letters rearranged gives 'osal'.
(Other definitions for proposal that I've seen before include "Plan, suggestion", "Offer, of marriage say", "put before the union?", "Suggestion put forward for consideration", "Offer (to get spliced?)", "An offer as of marriage", "Offer - suggestion", "Suggestion (of marriage?)", "A suggestion, perhaps of marriage" and "motion".)