Dull soldier, donning shorts, riding horse (8)
I believe the answer is:
sluggish
'dull' is the definition.
(synonyms)
'soldier donning shorts riding horse' is the wordplay.
'soldier' becomes 'GI' (term for US soldier).
'donning' indicates putting letters inside (I've seen this in other clues).
'shorts' becomes 'slugs' (both can mean alcoholic drinks).
'riding' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'horse' becomes 'H' (abbreviation as in horse power).
'gi' going inside 'slugs' is 'sluggis'.
'sluggis'+'h'='SLUGGISH'
(Other definitions for sluggish that I've seen before include "Slow moving" , "Hardly speeding" , "Lethargic" , "like a bullet?" , "Slow-moving, inactive" .)