Unwind the flexes with plug in (7,2)
I believe the answer is:
lighten up
'unwind' is the definition.
(unwind can mean to relax or lighten up)
'the flexes with plug in' is the wordplay.
'flexes with' indicates anagramming the letters.
'plug'+'in'='plugin'
'the' mixed up with 'plugin' makes 'LIGHTEN UP'.
(Other definitions for lighten up that I've seen before include "Plug in the fire" , "Unwind" , "Relax!" , "Chill" .)