Heading north is popular, depending on conditions at court (4)
I believe the answer is:
nisi
'depending on conditions at court' is the definition.
I can't judge whether this defines the answer.
'heading north is popular' is the wordplay.
'heading north' says the letters should be written in reverse.
'popular' becomes 'in' ('in' can mean trendy).
'is'+'in'='isin'
'isin' reversed gives 'NISI'.
Can you help me to learn more?
(Other definitions for nisi that I've seen before include "Kind of divorce decree" , "'De mortuis nil .... bonum.'" , "Unless - in Latin" , "(Of decree) taking effect when conditions are met" , "(Of decree) valid if certain conditions are met" .)