Treated gout, having singular enthusiasm (5)
I believe the answer is:
gusto
'enthusiasm' is the definition.
(I know that gusto is a type of enthusiasm)
'treated gout having singular' is the wordplay.
'treated' indicates anagramming the letters.
'having' means one lot of letters goes inside another.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'gout' with letters rearranged gives 'guto'.
'guto' placed around 's' is 'GUSTO'.
(Other definitions for gusto that I've seen before include "Spirit" , "Zest, pleasure in action" , "Vigorous enthusiasm" , "Got us with great enjoyment" , "Great relish" .)