Immature creature, bit extreme (7)
I believe the answer is:
tadpole
'immature creature' is the definition.
'tadpole' can be an answer for 'creature' (tadpole is a kind of animal). I'm not sure about the 'immature' bit.
'bit extreme' is the wordplay.
'bit' becomes 'tad' (both can mean a small amount).
'extreme' becomes 'pole' (I can't justify this - if you can you should believe this answer much more).
'tad'+'pole'='TADPOLE'
Can you help me to learn more?
(Other definitions for tadpole that I've seen before include "Little creature in water" , "Frog in formation" , "immature sea squirt" , "Young toad" , "being hatched in a puddle?" .)