Racehorse lacking a run will be unstable (7)
I believe the answer is:
trotter
'racehorse' is the definition.
(I know that trotter is a type of racehorse)
'lacking a run will be unstable' is the wordplay.
I cannot quite understand how this works, but
'run' could be 'trot' (trot is a kind of run) and 'trot' is present in the answer.
The remaining letters 'ter' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for trotter that I've seen before include "Edible foot?" , "Steady horse; pig's foot" , "horse getting sulky, say" , "Food for man" , "Pig's foot, or one on the run" .)