Catch brother or sister pinching postman's bottom (4)
I believe the answer is:
snib
'catch' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?
'brother or sister pinching postman's bottom' is the wordplay.
'brother or sister' becomes 'sib' (I've seen this in another clue).
'pinching' is an insertion indicator (taking inside).
'bottom' suggests the final letters.
The final letter of 'postman' is 'n'.
'sib' going around 'n' is 'SNIB'.
Can you help me to learn more?
(Another definition for snib that I've seen is " Fasten".)