Steady partner for person in the same club (10)
I believe the answer is:
stablemate
'person in the same club' is the definition.
(stablemate can informally mean a colleague)
'steady partner' is the wordplay.
'steady' becomes 'stable' (associated in meaning).
'partner' becomes 'mate' (synonyms).
'stable'+'mate'='STABLEMATE'
'for' acts as a link.
(Other definitions for stablemate that I've seen before include "someone from same place" , "colleague in same organization" , "Nag" , "One from the same organisation as another" , "companion" .)