Becoming fat let us work out (8)
I believe the answer is:
tasteful
'becoming' is the definition.
(thesaurus)
'fat let us work out' is the wordplay.
'work out' is an anagram indicator.
'fat'+'let'+'us'='fatletus'
'fatletus' is an anagram of 'TASTEFUL'.
(Other definitions for tasteful that I've seen before include "Aesthetically pleasing" , "Refined, not tawdry" , "Discriminating" , "Let Faust have ability to enjoy good things" , "pleasantly entertaining" .)