Mere lies can be more becoming (8)
I believe the answer is:
seemlier
'more becoming' is the definition.
('seemly' can be a synonym of 'becoming')
'mere lies can be' is the wordplay.
'can be' is an anagram indicator.
'mere'+'lies'='merelies'
'merelies' anagrammed gives 'SEEMLIER'.
(Other definitions for seemlier that I've seen before include "More suitable" , "Melee, sir, may be more proper" , "More meet" , "More becoming or decent" , "More in accordance with good taste" .)