Master caught opening his trap (5)
I believe the answer is:
mouth
'trap' is the definition.
(I've seen this before)
'master caught opening his' is the wordplay.
'master' becomes 'm'.
'caught' becomes 'out'.
'opening' indicates taking the first letters.
The first letter of 'his' is 'h'.
'm'+'out'+'h'='MOUTH'
(Other definitions for mouth that I've seen before include "Gob" , "Oral cavity" , "trap" , "Opening; say" , "Where a river enters the sea" .)