King perched up for assignment (4)
I believe the answer is:
task
'assignment' is the definition.
(I've seen this before)
'king perched up' is the wordplay.
'king' becomes 'K' (chess abbreviation).
'perched' becomes 'sat' (perch can mean to take a seat).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'k'+'sat'='ksat'
'ksat' back-to-front is 'TASK'.
'for' is the link.
(Other definitions for task that I've seen before include "Set piece of work" , "Errand, duty" , "something to do" , "Burden with work" , "Chore, job." .)