See the one left inside is ruined (6)
I believe the answer is:
Here is my best explanation:
'is ruined' is the definition.
'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.
'spot' placed around 'il' is 'SPOILT'.
(Other definitions for spoilt that I've seen before include "Made a mess of", "Overindulged", "Destroyed the value of", "ruined", "Indulged", "injured", "messed up", "Marred", "Deprived" and "Went off".)