Motion forward, also deviating (8)
I believe the answer is:
proposal
'motion' is the definition.
(thesaurus)
'forward also deviating' is the wordplay.
'forward' becomes 'prop' (I've seen this before).
'deviating' indicates anagramming the letters.
'also' anagrammed gives 'osal'.
'prop'+'osal'='PROPOSAL'
(Other definitions for proposal that I've seen before include "a match might be arranged?" , "Offer (to get spliced?)" , "union offer?" , "A suggestion, perhaps of marriage" , "Offer, of marriage say" .)