The one place that's perfect (5)
I believe the answer is:
ideal
'perfect' is the definition.
(similar in meaning)
'the one place' is the wordplay.
'the one' becomes 'i' (Roman numeral).
'place' becomes 'deal' (I can't justify this - if you can you should give a lot more credence to this answer).
'i'+'deal'='IDEAL'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "Optimum" , "Benchmark" , "Something that one hopes to attain" , "Perfectly suited" , "Cutting/stabbing weapons" .)