Say no more, level diminished (6)
I believe the answer is:
shrank
'diminished' is the definition.
(shrinking is a kind of diminishing)
'say no more level' is the wordplay.
'say no more' becomes 'sh' ('sh' means stop talking).
'level' becomes 'rank' (I've seen this before).
'sh'+'rank'='SHRANK'
(Other definitions for shrank that I've seen before include "Shrivelled" , "Got less" , "Flinched" , "Recoiled" , "Reduced in size" .)