All certain it's healthy (9)
I believe the answer is:
wholesome
'healthy' is the definition.
(associated in meaning)
'all certain' is the wordplay.
'all' becomes 'whole' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'certain' becomes 'some' (synonyms).
'whole'+'some'='WHOLESOME'
'it's' is the link.
Can you help me to learn more?
(Other definitions for wholesome that I've seen before include "Moral" , "Previously propitious" , "Healthy in body, taste or morals" , "Beneficial" , "Good for you!" .)