Catching by knocking right inside (8)
I believe the answer is:
trapping
'catching' is the definition.
(trapping is a kind of catching)
'by knocking right inside' is the wordplay.
'by' says to put letters next to each other.
'knocking' becomes 'rapping' ('rap' can be a synonym of 'knock').
'right inside' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'rapping' put after 't' is 'TRAPPING'.
Can you help me to learn more?
(Other definitions for trapping that I've seen before include "Snaring" , "Ensnaring" , "Catching in a snare" .)