Monsters consequently turning on head of security (5)
I believe the answer is:
ogres
'monsters' is the definition.
(ogre is a type of monster)
'consequently turning on head of security' is the wordplay.
'consequently turning' becomes 'og' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' becomes 're' ('on' can be mean 'with reference to').
'head of' indicates taking the first letters.
The initial letter of 'security' is 's'.
'og'+'re'+'s'='OGRES'
Can you help me to learn more?
(Other definitions for ogres that I've seen before include "Bugbears" , "Fairytale giants" , "Gorse (anag.)" , "Monsters can be goers" , "She-giant of fairy tale" .)