Clothing worn by mounted soldier going astray (10)

'going astray' is the definition.
(to digress is to go astray from a topic)

'clothing worn by mounted soldier' is the wordplay.
'clothing' becomes 'dressing' ('dress' can be a synonym of 'clothe').
'worn by' is an insertion indicator (outside letters are worn by those on the inside).
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'soldier' becomes 'GI' (term for US soldier).
'gi' reversed gives 'ig'.
'dressing' enclosing 'ig' is 'DIGRESSING'.