Spoil losing weight, getting evenly round figure (6)
I believe the answer is:
reckon
'round figure' is the definition.
'reckon' can be an answer for 'figure' (synonyms). I'm unsure of the 'round' bit.
'spoil losing weight getting evenly' is the wordplay.
'spoil' becomes 'wreck'.
'losing' is a deletion indicator.
'weight' becomes 'w' (short for weight).
'getting evenly' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
'wreck' with 'w' removed is 'reck'.
'reck'+'on'='RECKON'
(Other definitions for reckon that I've seen before include "Calculate or conclude" , "Consider, judge to be probable" , "Estimate - judge" , "Calculate, be of the opinion" , "Calculate, compute or estimate" .)