Deranged bloke holds one small dagger (7)
I believe the answer is:
Here is my best explanation:
'dagger' is the definition.
('obelisk' can be a synonym of 'dagger')
'deranged bloke holds one small' is the subsidiary indication.
'deranged' indicates anagramming the letters.
'holds' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'small' becomes 's' (abbreviation - e.g. clothes size).
'bloke' anagrammed gives 'obelk'.
'obelk' going around 'is' is 'obelisk'.
(Other definitions for obelisk that I've seen before include "Tapering pillar erected as monument", "Tapering square pillar", "Monumental pillar", "Needle-like column", "Tapering monumental pillar", "eg Cleopatra's needle", "Tapering pillar as monument or landmark", "Cleopatra's Needle, for instance", "e.g. Cleopatra's Needle" and "dagger".)