Unintentionally catch bird in undergrowth striking head (8)
I believe the answer is:
overhear
'bird in undergrowth striking head' is the definition.
I can't tell whether this defines the answer.
'unintentionally catch' is the wordplay.
'unintentionally' becomes 'over' (I am not sure about this - if you are sure you should believe this answer much more).
'catch' becomes 'hear' (to catch something is to hear it being spoken).
'over'+'hear'='OVERHEAR'
Can you help me to learn more?
(Other definitions for overhear that I've seen before include "Learn by accident" , "Accidentally perceive with the ear" , "Inadvertently catch" , "Pick up" , "Bug" .)