Choose to believe healthy (5,3)
I believe the answer is:
'choose' is the definition.
'believe healthy' is the wordplay.
'believe' becomes 'think' (synonyms).
'healthy' becomes 'fit' (associated in meaning).
'to' acts as a link.
(Other definitions for think fit that I've seen before include "Decide to do without good reason" , "Consider appropriate, choose to do" .)