Summit of Twitter these days following Apple regularly (4)
I believe the answer is:
apex
'summit' is the definition.
(I've seen this before)
'twitter these days following apple regularly' is the wordplay.
'twitter these days' becomes 'x' (I can't explain this - if you can you should give a lot more credence to this answer).
'following' means one lot of letters go next to another.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'apple' are 'ape'.
'x' after 'ape' is 'APEX'.
'of' is the link.
Can you help me to learn more?
(Other definitions for apex that I've seen before include "Summit - pinnacle" , "climax" , "Apogee, summit" , "Summit or peak" , "reasonable fare" .)