See the one left inside is ruined (6)
I believe the answer is:
spoilt
'is ruined' is the definition.
(thesaurus)
'see the one left inside' is the wordplay.
'see' becomes 'spot' (I've seen this before**).
'the one' becomes 'i' (Roman numeral).
'left' becomes 'l' (common abbreviation).
'inside' means one lot of letters goes inside another.
'i'+'l'='il'
'spot' placed around 'il' is 'SPOILT'.
(Other definitions for spoilt that I've seen before include "Destroyed the value of" , "Indulged" , "injured" , "Marred" , "messed up" .)