Gave nag a seat? (7)
I believe the answer is:
saddled
'gave nag a seat?' is the definition.
(put saddle on horse)
This is the entire clue.
(Other definitions for saddled that I've seen before include "Burdened with" , "Encumbered" , "Like a racehorse" , "Having to put up (with) - ready to be ridden" , "stuck" .)