Strengthen lock on entrances to building under threat (8)
I believe the answer is:
buttress
'strengthen' is the definition.
(buttressing is a kind of strengthening)
'lock on entrances to building under threat' is the wordplay.
'lock' becomes 'tress' (both can mean a piece of hair).
'on' is a charade indicator (letters next to each other).
'entrances to' says to take the initial letters (I've seen 'entrance to' mean this).
The first letters of 'building under threat' is 'but'.
'tress' after 'but' is 'BUTTRESS'.
(Other definitions for buttress that I've seen before include "Strengthen" , "Projecting support for building" , "Support built against a wall, prop" , "Stanchion" , "It strengthens the wall" .)