“Lacks stability,” they add (7)
I believe the answer is:
totters
'lacks' is the definition.
Although both the answer and definition are verbs in their -s form, I can't see how one could define the other.
'stability they add' is the wordplay.
I cannot quite understand how this works, but
'add' could be 'tot' ('tot' can be a synonym of 'add') and 'tot' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for totters that I've seen before include "More than one adder" , "salvage-collectors" , "Walks hesitantly" , "Staggers" , "rocks" .)