A job perched up on top of kirk (4)
I believe the answer is:
task
'a job' is the definition.
('task' can be a synonym of 'job')
'perched up on top of kirk' is the wordplay.
'perched' becomes 'sat' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The initial letter of 'kirk' is 'k'.
'sat' back-to-front is 'tas'.
'tas'+'k'='TASK'
(Other definitions for task that I've seen before include "Allocate work" , "exercise" , "Job to be done" , "commission" , "Piece of work to be done" .)