Support upright character in equine posture (10)
I believe the answer is:
assistance
'support' is the definition.
(I've seen this before)
'upright character in equine posture' is the wordplay.
'upright character' becomes 'I' (vertical letter of the alphabet).
'in' indicates putting letters inside.
'equine' becomes 'ass' (ass is a kind of equine).
'posture' becomes 'stance' (stance is a kind of posture).
'ass'+'stance'='assstance'
'i' going into 'assstance' is 'ASSISTANCE'.
(Other definitions for assistance that I've seen before include "Aid" , "Support" , "Help" .)