Declare a girl's put on weight (8)
I believe the answer is:
announce
'declare' is the definition.
(I know that declare can be written as announce)
'a girl's put on weight' is the wordplay.
'a girl' becomes 'ann' (woman's name).
'put on' says to put letters next to each other.
'weight' becomes 'ounce' (unit of weight).
'ann'+'ounce'='ANNOUNCE'
(Other definitions for announce that I've seen before include "Reveal" , "Disclose" , "Make known publicly or to an audience" , "Declare, proclaim" , "Formally declare" .)