Top-level sportsmen have the very thing for good health (6)
I believe the answer is:
prosit
'good health' is the definition.
(I've seen this before)
'top-level sportsmen have the very thing' is the wordplay.
'top-level sportsmen' becomes 'pros' (professional sportsmen as opposed to amateurs).
'have' means one lot of letters go next to another.
'the very thing' becomes 'it'.
'pros'+'it'='PROSIT'
'for' is the link.
(Other definitions for prosit that I've seen before include "Good health! Cheers!" , "I sport during German toast to your health" , "down the hatch" , "(German for) cheers!" , "Bottoms up (in Germany)" .)