Setter and Elsie, seen regularly together, initially go steady (9)
I believe the answer is:
stabilise
'steady' is the definition.
('steady'->'stabilize' is in my internal thesaurus and 'ize'->'ise')
'setter and elsie seen regularly together initially' is the wordplay.
'setter' becomes 'I' (the person who set this crossword).
'and' is a charade indicator (letters next to each other).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'together' becomes 'stab' (I can't explain this - if you can you should give a lot more credence to this answer).
'initially' means one lot of letters go next to another (some letters go before others).
'elsie'+'seen'='elsieseen'
The alternate letters of 'elsieseen' are 'lise'.
'i'+'lise' is 'ilise'.
'ilise' after 'stab' is 'STABILISE'.
'go' acts as a link.
Can you help me to learn more?
(Other definitions for stabilise that I've seen before include "Become less changeable" , "Make firm" , "Stop (something) rocking" , "Put into an equilibrium" , "regulate equilibrium" .)