Make it known the girl's gained little weight (8)
I believe the answer is:
announce
'make' is the definition.
(thesaurus)
'gained little weight' is the wordplay.
'gained' becomes 'ann' (I can't explain this - if you can you should give a lot more credence to this answer).
'little weight' becomes 'ounce' (unit of weight).
'ann'+'ounce'='ANNOUNCE'
'it known the girl's' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for announce that I've seen before include "Disclose" , "Broadcast (information)" , "Make known publicly or to an audience" , "Declare, proclaim" , "Reveal" .)