Cheer up to bleach (7)
I believe the answer is:
lighten
I believe this clue is a double definition.
'cheer up' is the first definition.
(to lighten up is to cheer up)
'bleach' is the second definition.
(thesaurus)
'to' acts as a link.
(Other definitions for lighten that I've seen before include "Thin leg (anag.)" , "make fairer" , "Make less burdensome" , "Ease burden or reduce weight" , "Make less serious" .)