Competent head of security is locking up at mill (12)
I believe the answer is:
satisfactory
'competent' is the definition.
(thesaurus)
'head of security is locking up at mill' is the wordplay.
'head of' suggests taking the first letters.
'locking up' is an insertion indicator (I've seen this in other clues).
'mill' becomes 'factory' ('factory' can be a synonym of 'mill'**).
The first letter of 'security' is 's'.
's'+'is'='sis'
'sis' enclosing 'at' is 'satis'.
'satis'+'factory'='SATISFACTORY'
(Other definitions for satisfactory that I've seen before include "unexceptional" , "fine" , "Acceptable - adequate" , "Fully acceptable" , "Meeting expectations" .)