Support also, surprisingly, the motion (8)
I believe the answer is:
proposal
'the motion' is the definition.
(thesaurus)
'support also surprisingly' is the wordplay.
'support' becomes 'prop' (prop is a kind of support).
'surprisingly' indicates an anagram.
'also' with letters rearranged gives 'osal'.
'prop'+'osal'='PROPOSAL'
(Other definitions for proposal that I've seen before include "A suggestion, perhaps of marriage" , "motion" , "Offer, of marriage say" , "Offer (to get spliced?)" , "Plan, suggestion" .)