Stick firmly to petition to keep every girl's clothing child-friendly (9)
I believe the answer is:
superglue
'stick firmly' is the definition.
'superglue' can be an answer for 'stick' (I've seen this before). I'm not certain of the 'firmly' bit.
'petition to keep every girl's clothing child-friendly' is the wordplay.
'petition' becomes 'sue' (both can mean to appeal or ask for something).
'to keep' means one lot of letters goes inside another.
'every' becomes 'per' ('per' means 'for each' or 'for every').
'clothing' suggests removing the centre.
'child-friendly' becomes 'u' (I can't explain this - if you can you should give a lot more credence to this answer).
'girl' with its middle removed is 'gl'.
'per'+'gl'+'u'='perglu'
'sue' placed around 'perglu' is 'SUPERGLUE'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for superglue that I've seen before include "that'll fix a lot of things!" , "Fast-bonding substance" , "solution" , "Substance used to stick" , "Extra strong adhesive" .)