Too early? Stop regularly (4)
I believe the answer is:
also
'too' is the definition.
('also' can be a synonym of 'too')
'early? stop regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'early'+'stop'='earlystop'
The alternating letters of 'earlystop' are 'ALSO'.
(Other definitions for also that I've seen before include "into the bargain" , "revolutionary a part for Yugoslavia" , "'Besides, moreover (4)'" , "Additionally" , "an indication of more to come" .)