A horse covered in fat is a slowcoach (7)
I believe the answer is:
laggard
'a slowcoach' is the definition.
(I've seen this before)
'a horse covered in fat' is the wordplay.
'horse' becomes 'gg'.
'covered in' means one lot of letters goes inside another.
'fat' becomes 'lard' (I've seen this before).
'a'+'gg'='agg'
'agg' put within 'lard' is 'LAGGARD'.
'is' acts as a link.
(Other definitions for laggard that I've seen before include "Dilatory person" , "See straggler" , "Drag gal around for one who falls behind" , "trailer" , "Slow mover" .)