Set up to keep brain active and lively (7)
I believe the answer is:
vibrant
'lively' is the definition.
(I've seen this before)
'set up to keep brain active' is the wordplay.
'set' becomes 'tv' (TV set).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'to keep' indicates putting letters inside.
'active' indicates anagramming the letters (the letters are active/moving).
'tv' written backwards gives 'vt'.
'brain' anagrammed gives 'ibran'.
'vt' enclosing 'ibran' is 'VIBRANT'.
'and' acts as a link.
(Other definitions for vibrant that I've seen before include "Brightly-coloured" , "Colourful, vigorous" , "Aquiver with life and excitement" , "Alive and energetic" , "Bright, thrilling or resonant" .)