Guarantee less than accurate reportedly (10)
I believe the answer is:
underwrite
'guarantee' is the definition.
(underwriting is a kind of guaranteeing)
'less than accurate reportedly' is the wordplay.
'less than' becomes 'under' (I've seen this before).
'accurate' becomes 'right' (similar in meaning).
'reportedly' indicates a 'sounds like' (homophone) clue.
'right' sounds like 'write'.
'under'+'write'='UNDERWRITE'
(Other definitions for underwrite that I've seen before include "Accept liability for" , "Guarantee financially" , "Sign" .)