Lamb done evenly — veg is burning (6)
I believe the answer is:
ablaze
'is burning' is the definition.
(I've seen this before)
'lamb done evenly veg' is the wordplay.
'done evenly' indicates alternate letters (letters in even-numbered positions).
'veg' becomes 'laze' (veg can mean to do nothing or laze around).
The alternating letters of 'lamb' are 'ab'.
'ab'+'laze'='ABLAZE'
(Other definitions for ablaze that I've seen before include "On fire - brightly lit" , "Burning fiercely" , "burnt" , "Bright" , "being toasted?" .)