Notice girl put on weight, intended to be understood (12)
I believe the answer is:
announcement
'notice' is the definition.
(I have seen 'Public notice ' mean 'announcement' so perhaps 'notice' could also mean 'announcement')
'girl put on weight intended to be understood' is the wordplay.
'girl' becomes 'Ann' (woman's name).
'put on weight' becomes 'ounce' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'intended' becomes 'meant' (to mean something is to intend it).
'to be understood' shows a homophone (sound like).
'meant' sounds like 'ment'.
'ann'+'ounce'+'ment'='ANNOUNCEMENT'
(Other definitions for announcement that I've seen before include "Statement, declaration" , "Public notice" , "Public declaration" , "Proclamation" , "report" .)