The others holding coach harness (9)
I believe the answer is:
restraint
'harness' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'the others holding coach' is the wordplay.
'the others' becomes 'rest' (eg the rest of a group).
'holding' indicates putting letters inside.
'coach' becomes 'train' (both can mean to instruct a sportsperson).
'rest' enclosing 'train' is 'RESTRAINT'.
Can you help me to learn more?
(Other definitions for restraint that I've seen before include "Reserve" , "controlling influence" , "Something that holds back" , "Moderation" , "Controlling measure" .)