Let bus become understated (6)
I believe the answer is:
subtle
'understated' is the definition.
(I've seen this before)
'let bus become' is the wordplay.
'become' is an anagram indicator.
'let'+'bus'='letbus'
'letbus' anagrammed gives 'SUBTLE'.
(Other definitions for subtle that I've seen before include "Delicate and understated" , "Delicate, difficult to describe" , "'Fine, elusive (6)'" , "Slight and difficult to analyse, as distinction may be" , "Delicate - crafty - indirect" .)