If fat, side salad could make you healthy (2,3,2,1,6)
I believe the answer is:
as fit as a fiddle
'you healthy' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'if fat side salad could make' is the wordplay.
'could make' is an anagram indicator.
'if'+'fat'+'side'+'salad'='iffatsidesalad'
'iffatsidesalad' with letters rearranged gives 'AS FIT AS A FIDDLE'.
Can you help me to learn more?
(Another definition for as fit as a fiddle that I've seen is " in form!".)