Rock on the radio for immature six-footer? (5)
I believe the answer is:
larva
'immature six-footer?' is the definition.
(I've seen this before)
'rock on the radio for' is the wordplay.
'rock' becomes 'lava' (lava is a kind of rock).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'the radio for' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'lava' enclosing 'r' is 'LARVA'.
Can you help me to learn more?
(Other definitions for larva that I've seen before include "Insect's immature form" , "Caterpillar perhaps" , "Grub of e.g. beetle" , "Maggot" , "Manage" .)