Haggard rogue has to swear nothing to us (10)
I believe the answer is:
cadaverous
'haggard' is the definition.
('cadaverous' can be a synonym of 'haggard')
'rogue has to swear nothing to us' is the wordplay.
'rogue' becomes 'cad' (I've seen this before).
'has' is a charade indicator (letters next to each other).
'to swear' becomes 'aver' ('aver' can be a synonym of 'swear').
'nothing' becomes 'o' (looks like zero - 0).
'cad'+'aver'+'o'+'us'='CADAVEROUS'
(Other definitions for cadaverous that I've seen before include "Resembling a corpse" , "Thin and bony like a corpse" , "not looking healthy" , "Gaunt" , "Unhealthy-looking" .)