Horse is steady and dull, last in race (10)
I believe the answer is:
stablemate
'horse' is the definition.
(stablemate is a kind of horse)
'steady and dull last in race' is the wordplay.
'steady' becomes 'stable' ('stable' can be similar in meaning to 'steady'**).
'and' says to put letters next to each other.
'dull' becomes 'mat' ('mat' can be similar in meaning to 'dull').
'last in' suggests the final letters.
The last letter of 'race' is 'e'.
'stable'+'mat'+'e'='STABLEMATE'
'is' is the link.
(Other definitions for stablemate that I've seen before include "Maybe fellow Arab" , "Blames Tate (anag) - person from the same club" , "colleague in same organization" , "someone from same place" , "Partner" .)