Old boy, this compiler, wearing belt to be secure (6)
I believe the answer is:
obtain
'be secure' is the definition.
(I've seen this in another clue)
'old boy this compiler wearing belt' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'this compiler' becomes 'I' (the person who is writing these clues).
'wearing' is an insertion indicator.
'belt' becomes 'tan' (both can mean to hit).
'i' going into 'tan' is 'tain'.
'ob'+'tain'='OBTAIN'
'to' acts as a link.
(Other definitions for obtain that I've seen before include "Get or lay hold of" , "Gain possession of" , "Get or receive" , "Come into possession of" , "Hold good" .)