Perhaps Caterpillar tractor's rear trapped in rock (5)
I believe the answer is:
larva
'perhaps caterpillar' is the definition.
'larva' can be an answer for 'caterpillar' (I've seen this before). I'm not sure about the 'perhaps' bit.
'tractor's rear trapped in rock' is the wordplay.
'tractor's rear' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'trapped in' is an insertion indicator.
'rock' becomes 'lava' (lava is a kind of rock).
'r' placed into 'lava' is 'LARVA'.
Can you help me to learn more?
(Other definitions for larva that I've seen before include "a developmental stage" , "Immature form of caterpillar" , "Early stage of creature" , "Manage" , "early form" .)