In very good health (3,2,1,6)
I believe the answer is:
fit as a fiddle
'in very good health' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fit as a fiddle that I've seen before include "In the pink" , "Extremely healthy" , "In very good health" , "Flourishing" , "Bouncing with health" .)