Stay on - most important (6)
I believe the answer is:
remain
'stay' is the definition.
(synonyms)
'on most important' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'most important' becomes 'main' (I've seen this before).
're'+'main'='REMAIN'
(Other definitions for remain that I've seen before include "Continue in position" , "Stay behind" , "stop behind?" , "Stay or continue to be" , "Stay (in a place)" .)