Reversal of plain boundaries not proceeding in upright way (2,3,5)
I believe the answer is:
on all fours
'not proceeding in upright way' is the definition.
(on all fours rather than standing upright)
'reversal of plain boundaries' is the wordplay.
'reversal of' is a reversal indicator.
'plain' becomes 'llano' (a grassy plain).
'boundaries' becomes 'fours' (cricket).
'llano' back-to-front is 'onall'.
'onall'+'fours'='ON ALL FOURS'
(Other definitions for on all fours that I've seen before include "Maybe crawling" , "using arms and legs" , "in a defensive position" .)