What you will do if you can (8)
I believe the answer is:
preserve
'you can' is the definition.
'preserve' can be an answer for 'can' (preserve in a can). I'm unsure of the 'you' bit.
'what you will do' is the wordplay.
'what you will' becomes 'pre' (I can't justify this - if you can you should give a lot more credence to this answer).
'do' becomes 'serve' (synonyms).
'pre'+'serve'='PRESERVE'
'if' acts as a link.
Can you help me to learn more?
(Other definitions for preserve that I've seen before include "Refuse to give up" , "defend" , "area where game is held" , "Keep alive, maintain" , "Maintain, protect from decay" .)