Swore a lawman will hold steady (8)
I believe the answer is:
affirmed
'swore' is the definition.
('affirm' can be a synonym of 'swear')
'a lawman will hold steady' is the wordplay.
'lawman' becomes 'fed' (fed can informally mean a federal agent).
'will hold' means one lot of letters goes inside another (inserted letters are held inside).
'steady' becomes 'firm' (similar in meaning).
'fed' enclosing 'firm' is 'ffirmed'.
'a'+'ffirmed'='AFFIRMED'
(Other definitions for affirmed that I've seen before include "Averred" , "Declared solemnly to be true" , "Stated as a fact" , "Endorsed" , "Stated emphatically" .)