No one's fool, but upset by love (7)
I believe the answer is:
passion
'love' is the definition.
(synonyms)
'no one's fool but upset' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'fool but' becomes 'sap' (synonyms. I am not sure about the 'but' bit.).
'upset' says the letters should be written in reverse.
'no'+'is'+'sap'='noissap'
'noissap' reversed gives 'PASSION'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for passion that I've seen before include "Intense emotion, anger or love" , "thing one really cares about" , "Strong feeling of intense emotion" , "Rage" , "Fervent love" .)