Girl on unbuttressed piles gets rocket (7)
I believe the answer is:
missile
'rocket' is the definition.
(missile is a kind of rocket)
'girl on unbuttressed' is the wordplay.
'girl' becomes 'miss' (synonyms).
'on' means one lot of letters go next to another.
'unbuttressed' becomes 'ile' (I can't justify this - if you can you should believe this answer much more).
'miss'+'ile'='MISSILE'
'piles gets' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for missile that I've seen before include "Projectile (sometimes guided)" , "Military projectile" , "weapon that's fired" , "One could get fired" , "Weapon that is propelled" .)