Dance for a year (4)
I believe the answer is:
prom
'dance' is the definition.
(I know that prom is a type of ball)
'for a year' is the wordplay.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'a year' becomes 'm' (I can't justify this - if you can you should give a lot more credence to this answer).
'pro'+'m'='PROM'
Can you help me to learn more?
(Other definitions for prom that I've seen before include "Henry Wood concert" , "Albert Hall concert" , "Classical music concert for which part of the audience stands" , "Concert; US college dance" , "Short walk" .)