Look - just back the ruddy horse! (6)
I believe the answer is:
gander
'look' is the definition.
(informal term for a look or glance)
'back the ruddy horse' is the wordplay.
'back' says the letters should be written in reverse.
'the ruddy' becomes 'red' (I can't justify this - if you can you should believe this answer much more).
'horse' becomes 'nag' (term for an old horse).
'red'+'nag'='rednag'
'rednag' in reverse letter order is 'GANDER'.
'just' acts as a link.
Can you help me to learn more?
(Other definitions for gander that I've seen before include "Male bird - look" , "Male goose; look (colloq.)" , "glance" , "Goose sauce is good for it" , "Looking at something - ranged (anag)" .)