Predatory insect, old boy? It is (6)
I believe the answer is:
mantis
'predatory insect' is the definition.
(I've seen this before)
'old boy? it is' is the wordplay.
'old boy?' becomes 'man' (old boy is a kind of man).
'it is' becomes 'tis'.
'man'+'tis'='MANTIS'
(Other definitions for mantis that I've seen before include "Predatory insect" , "It appears to be praying but is more likely preying" , "Large-eyed insect, looks as if it's praying" , "Room equipped for milking cows" , "Insect with limbs raised as if in prayer" .)