Relax, mostly understanding the reverse of a long down entry? (7,2)
I believe the answer is:
lighten up
'relax' is the definition.
(to lighten up is to relax or be less serious)
'mostly understanding the reverse of a long down entry?' is the wordplay.
'mostly' means to remove the last letter.
'understanding' becomes 'lights' (I've seen this before).
'the reverse of' says the letters should be written in reverse.
'a long down entry?' becomes 'pune' (I can't justify this - if you can you should believe this answer much more).
'pune' reversed gives 'enup'.
'lights' with its final letter removed is 'light'.
'light'+'enup'='LIGHTEN UP'
(Other definitions for lighten up that I've seen before include "Plug in the fire" , "Unwind" , "Relax!" , "Chill" .)