Sustain the objection to the lock (8)
I believe the answer is:
buttress
'sustain' is the definition.
(thesaurus)
'objection to the lock' is the wordplay.
'objection' becomes 'but' (as in 'no ifs, no buts').
'to the lock' becomes 'tress' (both can mean a piece of hair).
'but'+'tress'='BUTTRESS'
'the' is the link.
(Other definitions for buttress that I've seen before include "Structure providing support" , "It strengthens the wall" , "Projecting support for building" , "that goes flying round the cathedral?" , "She-goat" .)