After brief second, press 'one' endlessly for operator (7)
I believe the answer is:
surgeon
'for operator' is the definition.
(I've seen this in another clue)
'after brief second press one endlessly' is the wordplay.
'after' says to put letters next to each other.
'brief second' becomes 's' (common abbreviation; 'brief' suggests an abbreviation).
'press' becomes 'urge' (synonyms).
'endlessly' means to remove the last letter.
'one' with its final letter taken away is 'on'.
's' next to 'urge' is 'surge'.
'surge'+'on'='SURGEON'
(Other definitions for surgeon that I've seen before include "9 practitioner (through keyhole?)" , "incisive person" , "Ship's doctor" , "Naval doctor" , "Operating medical practitioner" .)