Plain looking back - old coats outside crawling (2,3,5)
I believe the answer is:
on all fours
'crawling' is the definition.
(I've seen this before)
'plain looking back old coats outside' is the wordplay.
'plain' becomes 'llano' (a grassy plain).
'looking back' says the letters should be written in reverse.
'old' becomes 'o' (abbreviation).
'coats' becomes 'furs' (fur is a kind of coat).
'outside' is an insertion indicator (some letters go outside others).
'llano' backwards is 'onall'.
'o' put within 'furs' is 'fours'.
'onall'+'fours'='ON ALL FOURS'
(Other definitions for on all fours that I've seen before include "Maybe crawling" , "in a defensive position" , "using arms and legs" .)