Speak boastfully to secure a post (8)
I believe the answer is:
baluster
'post' is the definition.
(thesaurus)
'speak boastfully to secure a' is the wordplay.
'speak boastfully' becomes 'bluster' (I've seen this before).
'to secure' is an insertion indicator.
'bluster' enclosing 'a' is 'BALUSTER'.
(Other definitions for baluster that I've seen before include "Pillar supporting a rail, may be rustable" , "Pillar supporting staircase rail" , "pillar of support" , "This could hold up rail" , "post" .)