Pause at second summit (4)
I believe the answer is:
stop
'pause' is the definition.
(I know that pause is a more specific form of the action break up)
'at second summit' is the wordplay.
'at' says to put letters next to each other.
'second' becomes 's' (common abbreviation).
'summit' becomes 'top' (synonyms).
's' next to 'top' is 'STOP'.
(Other definitions for stop that I've seen before include "port of call" , "Arrest" , "Part of organ" , "set of organ pipes" , "Withhold payment" .)