Catch piece that's knocked up (4)
I believe the answer is:
trap
'catch' is the definition.
(I know that trap is a more specific form of the action catch)
'piece that's knocked up' is the wordplay.
'piece' becomes 'part' (synonyms).
'that's knocked up' says the letters should be written in reverse.
'part' in reverse letter order is 'TRAP'.
(Other definitions for trap that I've seen before include "Lock" , "fault" , "It commonly has a role in speaking" , "Vehicle" , "The Web" .)