Stop agonising before one rejects these introductions (5)
I believe the answer is:
abort
'stop' is the definition.
(as in aborting a mission)
'agonising before one rejects these introductions' is the wordplay.
'introductions' indicates taking the first letters (the letters which introduce the word).
The first letters of 'agonising before one rejects these' is 'ABORT'.
(Other definitions for abort that I've seen before include "Call off (mission) because of problem" , "Bring further progress to an end" , "Bring to premature conclusion" , "[FINISH] prematurely" , "Miscarry" .)