Brave chap guarding borders of Thebes (5)
I believe the answer is:
gutsy
'brave' is the definition.
(thesaurus)
'chap guarding borders of thebes' is the wordplay.
'chap' becomes 'guy' (**).
'guarding' is an insertion indicator.
'borders of' suggests removing the centre.
'thebes' with its centre removed is 'ts'.
'guy' enclosing 'ts' is 'GUTSY'.
(Other definitions for gutsy that I've seen before include "Plucky" , "Bold" , "Courageous, spirited" , "Spirited; greedy" , "Brave" .)