Long, thick locks securing small ministers' houses (6)
I believe the answer is:
manses
'ministers' houses' is the definition.
(a manse is a house for a church minister)
'long thick locks securing small' is the wordplay.
'long thick locks' becomes 'manes' (mane can mean some thick hair).
'securing' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'manes' enclosing 's' is 'MANSES'.
(Other definitions for manses that I've seen before include "Ecclesiastical residences in Church of Scotland" , "Ecclesiastical houses in Scotland" .)