Old boy given time security guards dock (7)
I believe the answer is:
bobtail
'dock' is the definition.
(I know that dock can be written as bobtail)
'old boy given time security guards' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'given' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'security' becomes 'bail' (I've seen this in another clue).
'guards' means one lot of letters goes inside another (inserted letters are guarded).
'ob'+'t'='obt'
'obt' placed inside 'bail' is 'BOBTAIL'.
(Other definitions for bobtail that I've seen before include "Rag, tag and ..." , "Stump behind" .)